I recently ran into this article https://whatjewsbelieve.org/jews-for-jesus-messianic-jews-and-hebrew-christians-are-not-jews/
Here is a relevant portion of the article
" A rabbi in the later Middle Ages named the Hai Gaon, as quoted by
Aderet in Responsa, VII #292, stated that a Jew who converted out of
the faith was no longer a Jew. This view was shared by numerous
rabbis, which can be seen in the Responsa literature of Simon ben
Zemah of Duran, Samuel de Medina, Judah Berab, Jacob Berab, Moses ben
Elias Kapsali and others in the Middle Ages.
It can also be seen more recently in the Responsa of the Satmar Rov in
his Divrei Torah, Yoreh Deah #59, paragraph 5, as well as in the
Responsa of Rabbi Moshe Feinstein, Even Haezer Volume 4 Number 53.
The very famous rabbi, Moses ben Maimon, called Maimonides (the
Rambam), also wrote that if a Jew converted to Christianity, he or she
was no longer a Jew. See Maimonides, Hilchot Mamrim Perek 3, Halacha
1-3, as well as in Maimonides’ Mishneh Torah, Avodat Kochavim 2:5.
Rabbi Moses Isserles demanded a formal conversion back to Judaism for
those who had converted out, but then wanted to return. He demanded
ritual immersion (mikveh) and repentance before a court of three (Beit
Din). You will see this also in other Responsa literature: Radbaz,
Responsa III, 415; Moses Isserles to Yoreh Deah 268.12; and Hoffman,
Melamed Leho-il II, 84. "
And the article mentions that the Anusim are still Jewish because that was a forced conversion and involuntary, not like anything that would happen today.
I have to wonder where some of these sources would mean the same re atheism too. 'cos eg a bit later it uses aryeh kaplan a a source but his statement is more ambiguous. For example the article says
Furthermore, this is the attitude of many Orthodox rabbis today. In
his book ‘The Real Messiah,’ Rabbi Aryeh Kaplan, writing for the Union
of Orthodox Jewish Congregations of America and the Orthodox youth
group ‘National Conference of Synagogue Youth,’ wrote (on page 21):
"This brings us back to our original question: What can a Jew lose by
embracing Christianity? The answer is: Everything. Christianity
negates the fundamentals of Jewish faith, and one who accepts it
rejects the very essence of Judaism. Even if he continues to keep all
of the rituals, it is the same as if he abandoned Judaism completely.
A Jew who accepts Christianity might want to call himself a ‘Jewish
Christian,’ but he is no longer a Jew. He can no longer even be
counted as part of a Jewish congregation."
So to Aryeh Kaplan it sounds like the pinnacle of no longer being a Jew, is no longer being counted as part of a jewish congregation..
Well, some would say atheists can't be counted for a Minyan, https://www.torahmusings.com/2019/01/including-non-religious-jews-in-a-minyan-and-its-discontents/ though i've never heard anybody say they are no longer Jewish. So I wonder if any of the sources that article gives would exclude atheists from being Jewish also. Or if some of the sources it mentions don't go as far as to say the person is no longer Jewish.
But anyow judging by that article there do seem to be sources that view conversion to another religion as a special case and the person is no longer considered Jewish.