It is not entirely certain what happened to him that Yom Kippur, but there is a hint in his great book, “The Star of Redemption”.
He writes, “Anyone who has ever celebrated Yom Kippur knows that it is something more than a mere personal exaltation… or the symbolic recognition of a reality such as the Jewish people… It is a testimony to the reality of God that cannot be controverted.”
Note that the word “reality” comes twice in this passage.
One reality on Yom Kippur is God. The spiritual tremor that runs through Yom Kippur proves this reality.
We encounter Him in the prayers, the music, the mood, the atmosphere, the internal metamorphosis within us.
On Yom Kippur Rosenzweig, who had never ceased to believe in God, encountered something extra: the Jewish God.
The second reality he encountered that day was the reality of the Jewish people.
Yom Kippur is not the individual soul, though if one is away from a synagogue and has no alternative Yom Kippur is possible and necessary on one’s own. In normal circumstances, however, Yom Kippur is the Jewish people in dialogue with themselves as well as with God.
Rosenzweig’s decision to stay a Jew was not only because he encountered the Jewish God but because he found that a Jew cannot sever himself from his people.
One of Rosenzweig’s biographers calls him the Guide to Reversioners (ba’alei t’shuvah – people who came back). In a sense we are all reversioners on Yom Kippur, and what we discover is that we cannot completely break with God or the Jewish people.