Most Christians call it baptism. It is the rite of initiation into membership in Christ's body. Some people say it is irrevocable. Some say you can deny it later either by speaking the denial or by acting in a way that shows you do not take it seriously.
James White in his book
Introduction to Christian Worship (pp. 189-192) states the Bible gives five major metaphors used in conjunction with baptism.
1. Union with Christ (Romans 6:3-4) "Have you forgotten that when we were baptized into union with Christ Jesus we were baptized into his death? By baptism we were buried with him and lay dead, in order that, as Christ was raised from the dead in the splendor of the Father, so also we might set our feet upon the new path of life."
2. Secondly we are brought into the church. We become members by baptism. (1 Corinthians 12:13) "'For indeed,' Paul says, "we were all brought into one body by baptism in one Spirit.'" In Galatians 3:27-28 Paul uses both of these images, we are in union with Christ and all made equal together, no more divisions such as Jew and Greek, male and female, slave and free man. As we enter together into the church earthly divisions become secondary.
3. Thirdly baptism is a new birth, out of the womb, arising out of the water a new person. Jesus told Nicodemus "No one can enter the kingdom of God without being born of water and the spirit." This image has been incorporated into the baptismal rite.
4. Baptism is for the forgiveness of sins. We are washing clean. The water is a physical metaphor for a spiritual cleansing. Ananias states to Saul (Paul) in Acts 22:16 "Be baptized at once, with invocation of his name, and wash away your sins." 1 Peter 3:21 and Hebrews 10:22 speak of an outward washing and an inward cleaning. Acts 2:38 states clearly "repent and be baptized every one of you, in the name of Jesus the Messiah for the forgiveness of your sins."
5. Lastly baptism is where the Holy Spirit is given to the new convert. Some suggest that a separate rite should be used for the Baptism of the Holy Spirit. In some services the Holy Spirit is given as a separate part after baptism with water. Acts 2:38 continues "and you will receive the gift of the holy Spirit." I think the baptized Christian is given the gift of the Holy Spirit. God said he would go away so that he could send the Comforter. The disciples in the upper room that first received it did not receive water baptism, just the Holy Spirit. I have little problem with a separate rite if you want. But a Christian need only ask God in his closet and God will send the Holy Spirit and renew you again and again whenever you ask. This is a good gift of God, who wants to keep giving it.
White tells us there are other minor metaphors but these are five major ones. Can anyone think of any other Biblical metaphors associated with baptism?