The verse (Yermiyahu 17:13) states: “The Lord who is the hope (mikvah) of Israel.” The Talmud (Yuma 85b) explains that though the literal translation is that G-d is the source of Israel’s hope, it also means that G-d purifies Israel like a mikvah.
On this Reb Mendel of Kotzk explained: [1]
“Just as a person must be completely submerged in a mikvah without even a hair protruding outside of it, so too must a person be completely enveloped in G-dliness.”
[1] Emet Mkotzk Titzmach Pg. 75