Before Whom Do You Purify Yourselves
Teshuvah
Rebbi Akiva said, "Fortunate are you, the Jewish people. Before Whom do you purify yourselves, and Who purifies you? Your Father in Heaven." The Jewish people can rejoice that they are cleansed through teshuvah, which Hashem Himself encourages, and that it is Hashem Who awakens them to repent. He proves it from two pesukim. The first is, "I will sprinkle you with pure water and you will be cleansed" (Yechezkel 36:25). The second is, "The mikveh of Yisroel is Hashem" (Yirmiyah 17:13). The word mikveh literally means hope, but here it can also be understood as a ritual bath. Just as a mikveh purifies the ritually unclean, so too Hashem purifies the Jewish people.
Source: Yoma, Chapter 8, Mishnah 9