I'm not sure there's evidence for widespread secularism among past Jewish communities, unless one would count Hellenistic Jews as secular, but there is quite a bit of evidence for ranges of observance at least as far back as the Second Temple period.
Early Second Temple Period:
In Nechemiah 8 it says:
"The entire people assembled as one man in the square before the Water Gate, and they asked Ezra the scribe to bring the scroll of the Teaching of Moses with which the LORD had charged Israel.
On the first day of the seventh month, Ezra the priest brought the Teaching before the congregation, men and women and all who could listen with understanding.
He read from it, facing the square before the Water Gate, from the first light until midday, to the men and the women and those who could understand; the ears of all the people were given to the scroll of the Teaching...Ezra opened the scroll in the sight of all the people, for he was above all the people; as he opened it, all the people stood up. They read from the scroll of the Teaching of God, translating it and giving the sense; so they understood the reading...Nehemiah the Tirshatha, Ezra the priest and scribe, and the Levites who were explaining to the people said to all the people, “This day is holy to the LORD your God: you must not mourn or weep,” for all the people were weeping as they listened to the words of the Teaching.
He further said to them, “Go, eat choice foods and drink sweet drinks and send portions to whoever has nothing prepared, for the day is holy to our Lord. Do not be sad, for your rejoicing in the LORD is the source of your strength.”
The Levites were quieting the people, saying, “Hush, for the day is holy; do not be sad.”
Then all the people went to eat and drink and send portions and make great merriment, for they understood the things they were told.
On the second day, the heads of the clans of all the people and the priests and Levites gathered to Ezra the scribe to study the words of the Teaching.
They found written in the Teaching that the LORD had commanded Moses that the Israelites must dwell in booths during the festival of the seventh month,
and that they must announce and proclaim throughout all their towns and Jerusalem as follows, “Go out to the mountains and bring leafy branches of olive trees, pine trees, myrtles, palms and [other] leafy trees to make booths, as it is written.”
So the people went out and brought them, and made themselves booths on their roofs, in their courtyards, in the courtyards of the House of God, in the square of the Water Gate and in the square of the Ephraim Gate.
The whole community that returned from the captivity made booths and dwelt in the booths—the Israelites had not done so from the days of Joshua son of Nun to that day—and there was very great rejoicing.
He read from the scroll of the Teaching of God each day, from the first to the last day. They celebrated the festival seven days, and there was a solemn gathering on the eighth, as prescribed.
So we see here that during the time of Ezra and Nechemiah, there were many people who weren't aware of what we would call today basic Jewish traditions/commandments - the High Holidays. But upon hearing what they should have been doing all along, they weren't insolent about it, but showed real remorse. I think this is evident of a level of very basic observance - belief in Hashem, respect of the Torah, but not enough knowledge, and, it seems that until that gathering, not enough want of knowledge. The text was left for the scholars, Levites and Kohanim.
From the Levites' reaction, in knowing how to properly celebrate Rosh Hashanah, we see that they were more observant than the rest of the people.
Later, the people of Judea recommitted to observing the Torah to the fullest extent:
"...“And the rest of the people, the priests, the Levites, the gatekeepers, the singers, the temple servants, and all who separated themselves from the peoples of the lands to [follow] the Teaching of God, their wives, sons and daughters, all who know enough to understand,
join with their noble brothers, and take an oath with sanctions to follow the Teaching of God, given through Moses the servant of God, and to observe carefully all the commandments of the LORD our Lord, His rules and laws..." (Nechemiah 10:29-30)
But eventually, observance levels waned again, while Nechemiah went back to Persia. When he returned some years later, he saw that Shabbat observance levels were down:
"At that time I saw men in Judah treading winepresses on the sabbath, and others bringing heaps of grain and loading them onto asses, also wine, grapes, figs, and all sorts of goods, and bringing them into Jerusalem on the sabbath. I admonished them there and then for selling provisions.
Tyrians who lived there brought fish and all sorts of wares and sold them on the sabbath to the Judahites in Jerusalem.
I censured the nobles of Judah, saying to them, “What evil thing is this that you are doing, profaning the sabbath day!
This is just what your ancestors did, and for it God brought all this misfortune on this city; and now you give cause for further wrath against Israel by profaning the sabbath!”
When shadows filled the gateways of Jerusalem at the approach of the sabbath, I gave orders that the doors be closed, and ordered them not to be opened until after the sabbath. I stationed some of my servants at the gates, so that no goods should enter on the sabbath." (Nechemiah 13:15-19)
And though years prior they had committed to not marrying non-Jewish women, now they did this once more and now had children born of intermarriage who were much more non-Jewish, culture-wise:
"Also at that time, I saw that Jews had married Ashdodite, Ammonite, and Moabite women;
a good number of their children spoke the language of Ashdod and the language of those various peoples, and did not know how to speak Judean." (Nechemiah 13:23-24)
Even one of the sons of the Kohen Hagadol had married a daughter of Sanblat the Horonite.
Nechemiah did his best to continue to preserve the observance of Torah within Judea, but the levels continued to fluctuate over the years.
This phenomenon also appeared in the diaspora. For example, the Jewish garrison-community stationed on the Isle of Yeb (Elephantine) in Egypt famously had their own temple. While the Wikipedia entry notes that most likely they worshipped other deities besides Hashem there in the temple, the claim, according to Alter Welner in his book Asseret Hashvatim, is based on the fact that there are papyri letters describing one of the leaders of the community, Yadanyah, having ordered religious supplies both for the worshippers of Hashem and for the worshippers of other deities. This makes sense, considering that among the garrison there were also non-Jewish Aramaean officers who worshipped their own deities, so there isn't much room to say that the Jews were polytheists.
What it does tell us is that these Jews saw nothing wrong with having their own temple even when the Second Temple had already been erected in Yerushalayim. The most famous of the Yeb texts is a papyrus written by the same Yadanyah to the governor of Judea, writing to him to tell him that their temple had been destroyed by a raging Egyptian mob and asking him to help in getting permission from King Darius to rebuild the temple. Yadanyah mentions that he previously petitioned the nobles of Yerushalayim, but they ignored his letters.
Another papyrus is the "Passover Papyrus", which was sent by a man named Chananyah1 from Yerushalayim to the Yeb Jews, explaining to them how to observe Pesach. According to Prof. Aron Meir, this may be evidence that at the time, there were different traditions on how to observe Pesach, and this letter may be evident of an attempt by the Yerushalayim leadership to assert their tradition over other traditions throughout the Jewish world.
It seems that the reason the Yerushalayim leadership ignored the Yeb's pleas for support was that that was their chosen strategy in asserting a central worship center in the Temple in Yerushalayim.2
The Greek/Roman Period:
Famously, after the Greek conquest of Judea and other parts of the world, many Jews became Hellenized, combining Greek ideas with Judaism. This led to various levels of observance and eventually caused schisms among the Jews. For example, there had long been a tradition that a dead Kohen Gadol is succeeded by one of his sons. But during Greek times, the habit of buying out the K"G became a thing. Jason seems to have been the first to have done this. But at least Jason was of the K"G family. His successor was Meneleus, who seems to have not even been a Kohen!
Post the Chashmonean Revolt, Hellenism was still strong in Eretz Yisrael and other parts of the world, in particular the Galilee.
The Yerushalmi tells us that when Rabban Yochanan ben Zakai was younger, he was sent to be the posek of the Galilee. During his time there (eighteen years), he received only two questions, leading him to lament:
"O Galilee, O Galilee, you hate the Torah; in the end you shall be filled with wrongdoers!" (Shabbat 81b)
This appears to be evidence of the commonness of low-levels of halachic strictness in the Galilee, as opposed to the Judea area, although many mikvaot were found in the Galilee from around this time period, so purity laws seem to have still been strictly observed.
Indeed, we know from the Talmudic texts that at the time, there were three levels of halachic-based social statuses: Chaver, Ne'eman and Am Ha'aretz. The Chaver was trustworthy in adhering to all laws of purity and tithes (Trumot U'Maasrot), the Ne'eman only in tithes and the Am Ha'aretz untrustworthy.
Cults also started becoming a thing during this time, and there were several: Prushim, Tz'dokim, Baytusim, Esse'im, some form of the early Christians3, Kana'im, etc. Josephus wrote that in his youth, he went around and tried out the three main sects of his time:
"And when I was about sixteen years old, I had a mind to make trim of the several sects that were among us. These sects are three: - The first is that of the Pharisees, the second that Sadducees, and the third that of the Essens, as we have frequently told you; for I thought that by this means I might choose the best, if I were once acquainted with them all; so I contented myself with hard fare, and underwent great difficulties, and went through them all. Nor did I content myself with these trials only; but when I was informed that one, whose name was Banus, lived in the desert, and used no other clothing than grew upon trees, and had no other food than what grew of its own accord, and bathed himself in cold water frequently, both by night and by day, in order to preserve his chastity, I imitated him in those things, and continued with him three years." (The Life of Flavius Josephus, section 2)
It's possible he did this because it might have been a common thing among the noble-born youth of Judea; to experiment with the different sects.
Also around this time, Apikorsut started to become a thing, as Epicureanism reached Jewish communities. Famously, Perek Chelek in Sanhedrin deals with the Apikorsim:
"All of the Jewish people, even sinners and those who are liable to be executed with a court-imposed death penalty, have a share in the World-to-Come, as it is stated: “And your people also shall be all righteous, they shall inherit the land forever; the branch of My planting, the work of My hands, for My name to be glorified” (Isaiah 60:21). And these are the exceptions, the people who have no share in the World-to-Come, even when they fulfilled many mitzvot: One who says: There is no resurrection of the dead derived from the Torah, and one who says: The Torah did not originate from Heaven, and an epikoros, who treats Torah scholars and the Torah that they teach with contempt. Rabbi Akiva says: Also included in the exceptions are one who reads external literature, and one who whispers invocations over a wound and says as an invocation for healing: “Every illness that I placed upon Egypt I will not place upon you, for I am the Lord, your Healer” (Exodus 15:26). By doing so, he shows contempt for the sanctity of the name of God and therefore has no share in the World-to-Come. Abba Shaul says: Also included in the exceptions is one who pronounces the ineffable name of God as it is written, with its letters."
Mendel Wohlman in Mistarei Ha'agadah believed there was enough evidence in the Talmud and midrashim to suggest that even some of the greatest sages were influenced by gnostic teachings. If this is true even just to some extent, then I would say that kal va'chomer other Jews. This means that there arose people in the Jewish communities who may have been observant of halacha but in terms of faith, were no longer strictly classically Jewishly monotheistic, which holds that Hashem is involved in this world, but held that Hashem was detached from this world and another powerful entity dealt with this world.
The Early Exile Period:
From around the 3rd-7th centuries, we have evidence of usage of polytheistic motifs in synagogues and tombs (the latter, mostly in the 2nd and 3rd century). For example, what appears to be the image of Helios in a synagogue mosaic from the 6th century. Arguably, this would be the equivalent of having a low or transparent mechitzah in a Modern Orthodox shul nowadays - not out of an active anti-Rabbinic mentality but something that just seems alright.
Around the world, there were communities with different levels of observance: Haggai Mazuz in his essay "Northern Arabia and Its Jewry in Early Rabbinic Sources: More than Meets the Eye" discusses Chazalic references to Arabian Jewry. He notes that Rekem, one of these communities, had many converts who were not well-versed in the law:
"Theoretically, it is also possible that some Jews from the Land of Israel were married to inhabitants of ha-Reqem and ha-Ḥegger; this might explain why a bill of divorce would be sent from there. But why must a courier who delivers such a document from ha-Reqem and ha-Ḥegger declare that it had been written and signed before him? The apparent answer is that some people in Reqem (and therefore, most likely, also in Ḥegger) were proselytes and thus were not well versed in the laws. The Mishna (Nīdda 7:3) implies as much: “All stains from Reqem are pure and R. Yehuda pronounces them impure because they are proselytes and mistaken.”"
But on the other hand, he wrote that it's possible, even quite likely, that the following three Tannaim were of the Arabian communities: Shimon Hateimani, Yehudah ben Teima and Yehudah ben Chagrah, which is evidence for at least enough connection between these communities and the central ones in Eretz Yisrael and Bavel that some of their youths would have aspirations to become talmidei chachamim and would even set out to said central communities to accomplish these goals (a low-level of observance and weak ties to the central authorities would more likely result in apathy toward deeper levels of Torah learning).
There's also some evidence that it took time for the Talmud to spread around the world:
In Bikoret L'toldot Hakaraim, Avrom Ber Gotlober wrote:
"And it appears to be true and in agreement with the happenings of the world, that for all time there were people in the nation, without being a special cult, who doubted the traditions and laws that were kept in the nation according to the tradition preserved from the fathers to the sons, and would not say "[that it] holy" to all that the nation would say "[that is] holy". And so too later, when arose the sages of every age...to set their righteous teachings at the time as a statute of Torah...there were found among the people of the nation smart and wise men against them who didn't understand the actions of the sages and what great urgency there was in them. These are people of short-sightedness, out of their love of the basic understanding [הפשט הפשוט]...and they object in their heart to the actions of the sages...and despite this, they were one people and one congregation and would not split apart, so long as the Talmud did not receive the sanctification of the Torah."
And on this he adds in a footnote:
"My master and my guide, the wise Rabbi RY"B zt"l in his book Beit Yehudah ch. 55...proved with strong, clear proofs that there were a number of d'rabanan commandments that weren't so widespread in the whole diaspora at once, but became so slowly, and even in the time of R' Yehudai Gaon (the time of Anan) not all of the d'rabanan commandment were yet kept in all of the diaspora...and greater than this we find that in the issue of tefillin that still hadn't spread throughout the entire world and the Yochasin brings that R"Y Baruch of Germaiza4, author of the Terumah...came to Spain and rebuked them about the mitzvah of tefillin. And it is written in Halachot Psukot of R"Y Gaon (Hilchot Tefillin): "In the issue of an amputee [חגר], should he put on tefillin in the time of the tefillah and kriyat shema? Or perhaps the only one who puts on tefillin is a great man [גברא רבא], and whoever is not a great man of the highest level doesn't put on because it looks like arrogance for the entire congregation doesn't put [tefillin]." (pg. 16-17)5
And later he writes:
"...I will not hide the fact that it came to my mind that it's possible that that the Tiberian vowel points [Nikud Teverani] or as the Rabbi Shada"l found, "Nikud Tavriya", isn't a reference to the city of Teveriya in Eretz Yisrael...but to the country of Crimea which was prior called Tavriya (Taurien oder Chersonnesus Taurice heute die Krin), as it is still called today in the language of Russia. And it is known that in the island of Crimea there were Jews from the time of the Second Temple...for without this [the hypothetical coming of the Karaites to Crimea], the Jews there were far from the home of the Talmud...and they didn't know of the yeshivot in Bavel..." (pg. 143-144)
And on the subject of Eldad Hadani, he quotes Julius Fürst:
"...He [Fürst] said in tastefulness that Eldad Hadani wasn't a Karaite scoundrel and cheat, but one of the free Jews that are in the mountains of Khorasan, who for ages didn't know of the Talmud but from the Tanach and a little besides.
On Yosef Dhu Nuwas, Jewish king of Khimyar, Dr. Paul Yule, one of the archeologists who dug around Khimyar, said when interviewed by Haggai Mazuz:
"Dhu Nuwas was involved in a Jewish culture that lacked any Talmudic influence, a culture that is vastly different from what we have today."
So here we have some evidence of a number of diaspora communities that had low-levels of observance, at least when compared to the more scholarly and better-established communities, such as the one in Bavel or had entirely different traditions altogether.
The Gaonaic Period:
Besides for the above quote from R"Y Gaon, the Gaonaic period most famously ushered in the Karaite sect. [Note: the following section on the Karaites is based on the entirety of the book Bikoret L'toldot Hakaraim] The Karaites first settled in Jewish communities around Eretz Yisrael and later joined diaspora communities. The presence of Karaites in various communities significantly changed the observance demographics of those communities: Suddenly, there were groups within the community who didn't believe in the Oral Torah (or at least, not the classic Rabbinical tradition of the OT). During what some 19th century Rabbinical and Karaite historians referred to as "the second Karaite era", the Karaites, still a fairly small sect, led by Binyamin Hanahawandi, shifted radically towards a more gnostic-like belief, which essentially created a resurgence of the Gnosticism of the mid-Mishnaic period. Luckily, though, it seems that this time the phenomenon was relegated only into that portion of the community; the Karaite portion, that is, as the Karaites were fierce opponents of the Rabbinical Jews, argued by those same historians to have been worse in their schisms than the various cults of the Second Temple period. The Nahawandi Gnostic era passed soon after Nahawandi's passing, ushering in the third Karaite period, which returned to classical monotheism but featured even harsher denials of various Talmudic traditions (Anan ben David, founder of the Karaites, had still maintained some Talmudic traditions). This third era featured some of the fiercest debates between Karaites and Geonim, including the ones between Rasag and Karaite leaders. The key difference between the Second Temple sects polemics and the Rabbinical/Karaite polemics is that though the Karaites separated themselves from the Rabbis much more than the Tzdokim or did from the Prushim, when tempers flared between the Rabbis and the Karaites, the most they did was write very harshly against the other group, unlike in the Second Temple era, which included libels to the Greeks and Romans.
This is the information I have for the time being, not having looked enough into the Rishonim and early Achronim period (prior to the rise of Reform). I believe, however, that on one hand, the fact that there were different sorts of Jews in the communities for many centuries is reflected also in later centuries, but on the other hand, the Rishonim period brought on several things that may have significantly elevated average observance levels: a. the spreading of great rabbis throughout the world, in particular to the European communities who up until the late Gaonaic period were pretty secluded and neglected. b. the creation of rabbinical works such as Mishneh Torah and Rashi's commentaries that helped even the average Joe (average Jew?) be able to start tackling learning Torah and Halacha.
One thing I can say, though, is that during the Rishonim period there came the creation of a new sect of Jews, the Anusim, Jews who were forced to convert to Christianity or Islam but opted to try and keep their Judaism in secret. The nearly-impossible feat led to many families losing various traditions and having to transgress many things. When some of these families attempted to return to their roots, oftentimes they were shunned. Lack of knowledge + little-to-no-connection with the local Jewish community may have resulted in Jews who differed greatly in observance levels.
The Achronim period shortly followed the Spanish Exile which created a significant number of new Anusim families that in later centuries attempted to reintegrate into the Jewish world, some more successful but many not so successful. On the other hand, the printing press had been invented, allowing for more spreading of information and upping the levels of literacy. However, this eventually led to the rise of the Haskalah movement which eventually brought upon the Reform, Conservative and Orthodox split.
1 There are speculations on who this Chanayah might be. Of the opinions, one is that he is Chanani, brother of Nechemiah. Another is that he is Chananyah, the captain of the fortress.
2 If that's the reason they never replied, then retrospect, perhaps not the wisest decision. The Yeb community eventually received permission to rebuild the temple, though with added prohibitions from the government, as to not incite the Egyptians once again. Eventually the temple was destroyed once more and not rebuilt again.
3 As the NT is clearly not objective in its descriptions of JC and his followers and there are disagreements about whether or not any one of the Yesh"us of the Talmud are the same as JC, it's unclear what really happened at the time.
4 According to Hebrew Wikipedia, this title was wrongly given to the R"Y Baruch because he was from France. The confusion is likely due to R"Y Baruch, father of the Maharam of Rothenburg, who was from Worms.
5 Interestingly, this tradition of having only important people putting on tefillin is similar to what Rabbi Sharon Shalom recounts on the tefillin equivalent of the Ethiopian community, which was worn only by important people.
