Rabbi Akiva said: Happy are you, Israel! Who is it before whom you become pure? And who is it that purifies you? Your Father who is in heaven, as it is said: “And I will sprinkle clean water upon you and you shall be clean” (Ezekiel 36:25). And it further says: “O hope (mikveh) of Israel, O Lord” (Jeremiah 17:1–just as a mikveh purifies the unclean, so too does he Holy One, blessed be He, purify Israel.

—Mishnah, tractate Yoma 8:9