He shall bathe his entire flesh in water. (Leviticus 15:16)

The Talmud (Eruvin 4b) explains that this refers to a mikvah (pool) of at least 40 se’ah of water (approximately 120 gallons). What rationale can be given for mikvah purification?

There once was a woman who was considering becoming a convert to Judaism. She contacted a rabbi, who informed her of the requirements, and also mentioned immersion in a mikvah. She was ready to do everything except for the mikvah ritual, which she found difficult to comprehend. He advised her to write about her dilemma to the Lubavitcher Rebbe.

She received the following response: According to the Talmud,1 a person who undergoes conversion is considered a newborn. When an embryo is formed in the mother’s womb, it is in a placenta surrounded by water on all sides. Similarly, the convert immerses himself entirely in the waters of the mikvah, and emerges a newborn person.

The Rebbe’s reply provides an insight into mikvah purification in general. Every person has a pure soul at birth. Afterwards, he may do things which defile his soul. Immersion in the mikvah is a form of rebirth, and through it he returns to his original state of purity.

The famed commentator Shaloh writes: “When one immerses in the mikvah, he should recite the verse, ‘A heart that is pure create for me, O G‑d,’2 because through immersion he becomes a newly created person.”

The first letters of the Hebrew words “pure create for me” (טהור ברא לי) spell out the word “immerse” in Hebrew (טבל).

According to an interpretation given in Kesef Mishneh,3 a person who immerses in a mikvah becomes pure when he first emerges from the water, and not while he is actually immersed. This ruling may reflect the spiritual correspondence between immersing in a mikvah and birth, for a child is considered born when his forehead emerges from his mother’s womb.