Brides in ancient Israel went to the mikvah (pool) for ritual immersion in water as a preparation for the wedding ceremony. Washings have, traditionally, carried the thought of a change or a new life. For her, she will become a married woman, i.e., she will belong to a man as his wife. She will leave the authority of her father and will enter the authority of her husband.
The spiritual application points to Jesus as the groom and John as the “friend of the bridegroom” (John 3:27-29). John wasn’t the groom. He points out that Jesus is the one they must honor and to whom they must submit. The symbolism of the mikvah points to the Father who purifies or sanctifies his people. The bride goes to the water of purification in preparation for the wedding. Jesus said in Mark 16:16: “He who believes and is baptized (immersed, goes to the mikvah) will be saved; but he who does not believe will be condemned.” It is in baptism that we reckon our old selves dead to sin. We are not under the power, dominion, or authority of Satan at that point. We no longer serve sin but are alive. The Groom has cleansed us by the gift of His blood and a new beginning has commenced wherein we submit to a new master (Rom. 6:4-18). The marriage ceremony begins after the cleansing. It is true that the betrothal takes place beforehand. The consent and agreement have been reached and a promise has been granted but the immersion precedes the consummation of marriage.