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.