I've read that Avraham Avinu proselytized to anyone he encountered, encouraging them to acknowledge HaShem as the Master of the Universe and express gratitude to Him. Yet, it is generally understood that Judaism is not a proselytizing religion. Are there any rabbinic explanations as to why Judaism doesn't proselytize even though its great progenitor actually proselytized?
I think there is a subtle difference.
To proselyte is to ask/convince someone to convert to a religion. In the case of Avraham there was no Jewish religion, it was just an acknowledgment of G-d's existence.
Rambam in Mishneh Torah, Hilchos Avodah Zara 1:3 explains that everyone worshipped idols until Avraham came on the scene. Avraham had this life-changing epiphany whereby he came and called Hashem "Lord" (See Berachos 7b)
It was a sincere desire to bring the awareness of G-d into the world that drove Avraham. The midrash in Bereishis Rabbah 39:16 says precisely this:
וַיִּקְרָא בְּשֵׁם ה', מְלַמֵּד שֶׁהִקְרִיא שְׁמוֹ שֶׁל הַקָּדוֹשׁ בָּרוּךְ הוּא בְּפִי כָּל בְּרִיָה
"And he called the name of Hashem" - this teaches that he caused the Name of G-d to be called upon by all people.
Indeed, the Gemara in Menachos 53a writes equally,
אמר לה טובתי בל עליך איני מחזיק טובה אלא לאברהם יצחק ויעקב שהודיעוני תחלה בעולם
G-d said, "I give credit to Avraham, Yitzchak and Yaakov, who were the first to make Me known in the world.
The conversion that it is written that Abraham converted is not the same conversion that we are talking about today.
Abraham our father according to the book of the Khazari he belongs to a holy seed from which came the people of Israel, but it is clear that all those who converted during his time did not become Jews, and that is because there was no Judaism yet, and the only thing that existed was the 7 mitzvot of the sons of Noah, and indeed even today it is of great importance to spread the 7 Mitzvah of the sons of Noah according to Judaism. (And it is true that there are Sage midrashi who say that Abraham our father fulfilled the mitzvot of Sages, but it is clear that it is only him and not the one who converted)
Maimonides wrote that we should imitate Abraham. And, what did Abraham do? Abraham proselytized. He made converts. He sat in the front of his tent, waiting to teach about God to anvone willing to listen.
Therefore, it is not surprising that Jews used to proselvtize. In fact, it got so bad that Rome banned Jews from seeking converts. Everyone was proselytizing; there were even Buddhist in the city of Alexandria.
Nothing in Judaism is against proselytizing. So, why don’t Jews seek converts today? One reason is that Jews are taught that one not need to be Jewish to have a share in world to come. This is true. However, Maimonides makes it clear that non-Jews must observe the Noahide laws and, for the right reasons. There is another reason why Jews stopped making converts. Once mysticism was introduced into Judaism some Jews began to fancy that they were biologically superior to non-Jews. They feared bringing in non-Jews into their community may dilute the “blood.” Lastly, Jews didn't proselvtize out of fear. Not because not allowed.
My own personal view is that Jews should activity be seeking converts purely for academic reasons. Academically, you will agree that 2+2=4, and not 5. It is the same with the Torah. If your convinced that the Torah is true then why not share that wisdom with everyone else? If you found the cure to cancer you would publish it for all. It behooves you to want to share God’s Torah with the world.
Judaism is not a race. Judaism is a religion. The Torah is clear that there is one law for the convert and the born Jew. Abraham was a Gentile. Abraham was not a Jew, but a Noahide. God never "recreated" man since Adam (making a superior one called the Jew); all humans are identical. We all come from Adam and Eve. We are all identical. Both King David and the messiah descend from Ruth the convert. Ruth was a gentile who converted and Kings David and Solomon, as well as the Messiah descend from her. We see from here that gentiles have the same capacity to become the greatest leaders as those who are born to Jewish parents. We are all identical.
Thus, Jews should proselytize. Put differently, we are never commanded not to proselytize. There is no Torah law or rabbinical law barring us from proselytizing non-Jews.