In the Mishnah (Yoma 85b): Rabbi Akiva said, "How fortunate are you, Israel; before Whom are you purified, and Who purifies you? Your Father in Heaven, as it is stated (Ezekiel 36:25), "And I will sprinkle purifying water upon you, and you shall be purified." And it states (Jeremiah 17:13), 'The mikveh (ritual bath) of Israel is God.' Just as a mikveh purifies the impure, so too does the Holy One, Blessed be He, purify Israel."

And we can say [about this] that the expression, who, applies when we ask about something that is possible to understand. It is just that I don't know what it is, so I ask about it, who. But it is not applicable to ask, who is it, about something that is impossible to understand! So this is the explanation of the Mishnah, "How fortunate are you, Israel; before Whom are you purified: The explanation is that a person purifies himself and comes to this aspect before it is possible to ask, "Who is He?" And this aspect called, "Who," purifies you, "as it is stated, 'And I will sprinkle purifying water upon you, and you shall be purified.' And it states, 'The mikveh (ritual bath) of Israel is God.'" Since, in the same way that a person must immerse himself in a mikveh, so too must he immerse himself in the Name of His Existence, blessed be He. And it needs to be 'water in water,' as it was before the creation of the world, so that he purifies himself before, "Who." And this is the aspect of Who purifying him. And this is the intention of the mikveh, as a person must immerse himself in the four letters of His Existence, may He blessed and blessed be His name, so that there be 'water in water' (ed. see Zohar Bereishit 6:60). And it is found in the Midrash, "'And they shall take a red heifer to you' (Numbers 19:2), etc. Such did Rabbi Tanchuma open [his teaching], '"The righteous man lives blamelessly" (Proverbs 20:7) - that is Avraham; "happy are his children who come after him" - everything that Avraham did to the angels was done afterwards to his children. He said (Genesis 18:4), "Let a little water be taken and wash your feet" - so do his children sprinkle upon themselves from the ashes of the red heifer, immerse themselves and become pure. For Avraham was truly the kindness of the world.'" The explanation is that he gave to everyone everything they needed. And just like Avraham did kindness to the angels and purified them - as the angels [appeared as] three men, as is well known, such that they needed purification, so he purified with water which did not have any aspect of impurity, but was rather completely pure - so too, did he purify Israel with the same water at that time. It is written (Exodus 30:18-20), "Make a copper basin, etc. When they enter the Tent of Meeting they shall wash with water (it can be read literally as, the water shall be washed)." It is implied that the water itself needs washing. But ostensibly, how is it possible to wash water in water? But it is according to that which is found in the Gemara - water that has become impure and requires purification, we adjoin it to pure water. And this is [the meaning of] "the water shall be washed": That the lower waters must be adjoined to the higher waters. And this is the explanation of "adjoining water to water." And this is the explanation of, "before Who": Before, "Who," you purify yourselves; as they are waters that are completely pure. And these are the waters that Avraham gave to the angels to purify them. So any time a person purifies himself, he must purify himself with those waters, in order to adjoin 'water with water' - 'the mikveh of Israel is God' (referring to God's name, which is seen as combining the two waters). And the trait called, "Who," is what purifies you. And this is [the meaning of] "and they shall take a pure red heifer to you that has no blemish in it" - that there should be no lack in it, but that it should be completely perfect; "upon which a yoke has not come" - for it is from the world of freedom. And that is the aspect called, "'Who,' which purifies you." And that is the explanation of the verse, "The righteous man lives blamelessly" - this is Avraham who did kindness with everyone; "happy are his children who come after him" - just as he did kindness with everyone, in that merit, his children are also purified in the same waters.