Does it say anywhere in the ancient Sources that antisemitism preserves Judaism, by causing the survivors to cling even more closely to their traditions, as a reaction? There is a hint of that in Makkot 24a, but it is not a compelling interpretation of the text.
(I am not talking about thinking: "Antisemites are harassing me because I sinned, so I will sin no more." I am talking about thinking: "So these guys don't like Jews, eh? I'll SHOW them by being even more Jewish.")
Bavli Megila 14a
אמר רבי אבא בר כהנא גדולה הסרת טבעת יותר מארבעים ושמונה נביאים ושבע נביאות שנתנבאו להן לישראל שכולן לא החזירום למוטב ואילו הסרת טבעת החזירתן למוטב
To what can we compare Ahasuerus and Haman at this point?
To two men one of whom had a mound in the middle of his field and the other a ditch in the middle of his field.
The owner of the ditch said, I wish I coul buy that mound, and the owner of the mound said, I wish l could buy that ditch.
One day they met, and the owner of the ditch said, Sell me your mound, whereupon the other replied, Take it for nothing, and I shall be only too glad.
And the king removed his ring.
Rabbi Abba Bar Kahana said: This removal of the ring was more efficacious than forty-eight prophets and seven prophetesses who prophesied to Israel; for all these were not able to turn Israel to better courses, and the removal of the ring did turn them to better courses.
There is a very famous משך חכמה in פרשת בחוקותי that makes this point
והנה מעת היות ישראל בגויים ככה יחליף מצב הישראלי וקיומו בעמים כאשר עין המשכיל יראה בספר דברי הימים וזה לשתי סיבות לקיום הדת האמיתי וטהרתו ולקיום האומה, הנה דרך ההשגחה כי ינוחו משך שנים קרוב למאה או מאתים ואחר זה יקום רוח סערה ויפוץ המון גליו... עד כי ירוצו יברחו למקום רחוק ושם יתאחדו והיו לגוי יוגדל תורתם חכמתם יעשו חיל עד כי ישכחו היותו גר בארץ נכריה יחשוב כי זה מקום מהצבתו בל יצפה לישועת ה' ברוחניות בזמן המיועד שם יבא רוח סערה עוד יותר חזק יזכיר אותו בקול סואן ברעש יהודי אתה ומי שמך לאיש לך לך אל ארץ אשר לא ידעת. ככה יחליף מצב הישראלי וקיומו בעמים... כי כאשר ינוח ישראל בעמים יפריח ויגדל תורתו ופלפולו ובניו יעשו חיל, יתגדרו נגד אבותיהם... כאשר עינינו רואות בכל דור
if anyone wants to edit and translate please do..
but to summarize he says that since we went into golus and began to live between the goyim, there is a cycle that is necessary for our survival. every couple of years (100 200 or so) we get too comfortable in our surroundings and that leads to assimilation, and so Hashem causes anti-Semitism to spew until they start screaming at us "you jew" etc. to remind us who we really are.
It's a long piece that's famous for basically predicting the holocaust (he writes that jews are starting to live as if Berlin/Germany is Jerusalem and so strong winds [of anti-Semitism] will start blowing from there...)
I think I have an answer. Not an ancient quote, but better than that: All of Jewish history.
After the Exodus, a lot of Jews lapsed into idolatry, and clung to it tenaciously and enthusiastically for 1,000 years. When Rav Ashi dreamed of King Menashe, he asked him: If you are so wise, why did you worship idols? King Menasheh replied: If you had lived in my time, you would have done the same. You would have lifted the hems of your robes to run behind me to worship idols also. [Sanhedrin 102b]
Then, after the exile in Babylon, idolatry disappeared from Judaism, suddenly and completely, and has remained absent for the next 2,500 years, to this day. What happened?
My explanation: During the 1,000 years of idolatry, the Jews were essentially free and masters of their own house. They felt they could indulge in whatever appealed to them, as free people do. Those who found idolatrous rites colorful and attractive felt free to indulge in them. But for the next 2,500 years, the Jews were subjugated and persecuted – by the Persians, the Greeks, the Romans, the Christians, the Muslims, etc. They felt they HAD to keep their own traditions and not adopt any of the customs of their overlords, as an act of rebellion. They were even willing to die rather than be forced to indulge in idolatry or reject their traditions.
Here we have the evidence that antisemitism preserved Judaism.
As for quotes, I found something by Maharsha in Artscroll: When a fruit or vegetable falls on the ground, it rots. Yet the very ground which causes it to rot is the environment necessary for it to take root and grow and be reborn. So it is with Israel in exile: Although many Jews will die, the very hostile environment of persecution will cause their spiritual rebirth.
