A mikvah in the Hebrew Bible is a gathering or collection of water. The word came to refer to a pool of water used for ceremonial cleansing. One who is impure or ceremonially unclean before immersion will be pure or ceremonially clean after immersion in a mikvah. A person would have to be ceremonially clean before entering the temple. Ceremonial cleansing is prescribed in the Bible on a number of occasions: women after childbirth or their monthly cycle and men after sexual discharge (Leviticus 15:19–30) and after contact with a dead body (Leviticus 19:18–19). Clothing and utensils could also be cleansed by ritual immersion (Leviticus 11:32). Later, ritual immersion—baptism—became part of a proselyte’s conversion to Judaism.
Any natural body of water is considered a mikvah (Leviticus 11:36). Today, mikvahs are also constructed in homes. A mikvah must be built into the home, below ground. It cannot be portable. A mikvah must have contact with natural water, so mikvahs constructed in modern homes will normally have one primary pool filled with tap water and used for immersion, and a secondary pool where rain water or water from some other natural source is collected. A hole between the two pools will allow the water to mix, thus rendering the primary pool fit for ceremonial cleansing.
Today, Orthodox, Conservative, Reformed, and Reconstructionist Jews have different practices regarding the use of the mikvah, but the theme of spiritual purification and cleansing is common to all of them. Physical cleansing is not in view, as participants are supposed to be physically clean before entering the pool.
The word mikvah is from the same root word as hope, and there is some word play between the two in Jeremiah 17:13: “O Lord, the hope of Israel, all who forsake You will be put to shame. Those who turn away on earth will be written down, because they have forsaken the fountain of living water, even the Lord.” (Here, mikvah is translated “hope,” and then described as a “fountain of living water,” which was the requirement for a mikvah—naturally flowing water.)
The use of the mikvah, or ceremonial immersion, seems to provide the background for New Testament baptism. When John was baptizing and preaching repentance, those who responded were admitting that they were “unclean.” This seems to be why John objected so strongly to baptizing Jesus (Matthew 3:13–14) and why the Pharisees refused to be baptized (Matthew 3:7). Likewise, Christian baptism would have been understood against this background. Those who responded in repentance and baptism were admitting that they were unacceptable to God and needed to be made clean (see Acts 2:37–38).
Being immersed in a mikvah was a powerful symbol that the Jews of Jesus’ time would have understood. Jesus’ words to Nicodemus, “Very truly I tell you, no one can enter the kingdom of God unless they are born of water and the Spirit” (John 3:5), likely contain an allusion to the mikvah, which Nicodemus would have naturally apprehended. Jesus was saying that, in order to be right with God, a person must repent of sin (which is what the ritual cleansing in water signified) and undergo a spiritual transformation wrought by God Himself.
Unlike ritual immersion in a mikvah, which is a repeated ritual in the Old Testament and by modern Jews, Christian baptism is a one-time symbol of cleansing. In Christ, believers are made clean once for all, and a single baptism is the symbol for that.