In Deuteronomy 21:10ff., God gave His people this commandment: “If you see a beautiful woman among the captives and you want to take her as your wife, bring her into your house, let her hair be shaven and her nails trimmed, and her clothes in which she was taken captive laid aside, and let her sit in your house and mourn her father and her mother for one month. Then sleep with her and marry her and let her be your wife.”
Along with the fact that the Lord God wanted to use these various ceremonies in order to prevent the Israelites from entering into marriage with foreigners and idol worshipers lightly, there is an excellent type, or rather, an allegory presented to us in this ceremonial law. For the captive foreign woman stands for the soul of a sinner, which is a “foreigner and outside of the city and people of God” (Eph. 2:12). It is also captive in the kingdom of the devil (Col. 1:13). If she (that is, the soul) figuratively becomes pledged in marriage to Christ, the heavenly Bridegroom, then she must first allow herself to be washed and purified by true repentance.
She must “lay aside her clothing in which she was taken captive,” that is, she must “take off the old man” (Eph. 4:22) and “despise the stained garment of the flesh” (Jude 23).
She must “let her hair and nails be trimmed”; that is, she must lay aside the evil lusts of the flesh, especially haughtiness and greed, which are understood by the hair and nails, and she must undergo a spiritual circumcision of the heart (Deu. 30:6). (Sermon for Mary Magdalene-July 22, Postilla Volume 3 by Johann Gerhard consisting of 15 sermons for Saints Days and other festivals, Repristination Press, page 106)