Mikvah
The mikvah, the Jewish ritual bath, has an important place in the history of traditional Judaism, Christianity, and Messianic Judaism. God told Moses that Jews are to take a mikvah on quite an array of occasions: after contact with semenal fluid (Leviticus 15:16 and 18 and Deuteronomy 23:11) or menstrual blood (Leviticus 15:21, 22, and 27), upon being cured from certain skin diseases (Leviticus 14:8-9), after exposure to certain diseases (Leviticus 15:5, 6, 8, 10, 11, and 13), after contact with a dead animal (Leviticus 17:15-16) or human (Numbers 19:7-8, and 19), upon becoming a cohen (Exodus 29:4 and 40:12), and during the Yom Kippur ritual (Leviticus 16:4, 24, 26, and 28).
As you pursue these regulations and others like them, you may notice that there seem to be three general areas of life that require a mikvah. On any occasion involving birth, death, or the Presence of God, a mikvah is required. I take it that the mikvah is God's reminder to us that we are not like God since we are born and die and we must be cleansed to be in the presence of God. We do not have to be passively resigned to accept the filthiness and death which comes from our alienation from God. He has a bath for us, as effective as the one He gave us at the Red Sea when He saved us through water. (I Corinthians 10:2 says that all Israel became followers of Moses by their mikvah in the Red Sea after Passover.) They were baptized into Moses as we are baptized "into Christ." (Gal. 3:36-27)
Like many Jewish rituals, the mikvah is both a statement of faith and an acted-out prayer. Our washing ourselves in mikvah is our way of expressing faith that we have been cleansed. It is also an acted-out prayer asking Him for the cleansing and being assured we received it. Rav Sha'ul (the Apostle Paul), using this line of logic, argues for the truth of the coming resurrection from the biblical command to take a mikvah after touching a corpse, in the seldom-understood but easily-understandable verse, I Corinthians 15:29. "Otherwise, what should they do who take a mikvah concerning a corpse, if the dead do not rise at all? Why then would they take a mikvah concerning a corpse?"
The Torah gives little or no description of the mikvah, such as indoors or out, how much clothing to wear, how much water, whether to be totally immersed, etc. The emphasis is entirely on the situations that require a mikvah, and on its meaning. (Since the mikvah was taken outdoors in early times, one would naturally assume that the bather would be fully clad.)
When Alexander the Great conquered that part of the world, and Jews began speaking Greek, they needed a way to talk about the mikvah in Greek. There was a Greek word for bath, but the Greek bath was a hedonistic, secular experience, often involving the latest gossip, crude conversation, and homosexual licentiousness. So the Jewish People of that time did not use the Greek word for bath to talk about their ritual of purification, but the Greek word for dipping, "vaptismos". Historically, "vaptismos" transliterates into English as "baptism".
An interesting use of the mikvah in first-century Jewish customs involved becoming the student of a rabbi. There were no seminaries, only apprenticeships. If a rabbi accepted you as his student, he would supervise your taking a mikvah in his name. From that time on, you were considered cleansed from your old life of sin and ignorance and "born again" with your rabbi as your spiritual father. From then on, you are obligated to believe and observe everything that rabbi teaches you, and to obey the rabbi's every command. The rabbi, in turn, is obligated to teach you everything he knows about God and about God's Word. That's the cultural background of most uses of the concept of the mikvah in the Renewed Covenant Scriptures.
As Christianity lost some of its Jewishness and became more of a separate religion, baptism became less a commitment to follow our Rabbi and became more of an initiation into the body of Christ.