A sacrament is an outward sign of an inward truth. It is an act of faith that Christians do on the outside to celebrate what God has done for them on the inside.
Baptism has been a symbolic way of joining the Church from the very start of Christianity.
It actually predates Christianity; John the Baptist baptized Jews before Jesus came on the scene, and so John baptised Jesus even before the Church existed.
Water is used in baptism, and is a symbol of washing away sin and the start of a new life.
Jesus told his disciples that they were to baptise those who came to follow him.
Initially it seems that it was adults who believed in Jesus who were baptised, but over time the baptising of whole families, including infants began.
In the New TestamentThe second part of the Christian Bible containing 27 books consisting of four types of literature: letters (epistles), gospels, history and prophecy. book of Acts, the Apostle Paul baptised the Philippian jailer and his whole household. Some have argued that the household must have included infants.
In the New Testament book of Colossians Paul writes:
In him you were also circumcised with a circumcision not performed by human hands. Your whole self ruled by the flesh was put off when you were circumcised by Christ, having been buried with him in baptism, in which you were also raised with him through your faith in the working of God, who raised him from the dead.
— Colossians 2:11-12
It has been argued that this passage links baptism to circumcision (Judaism)The removal of the foreskin from the penis performed by a qualified mohel on all Jewish boys, usually on the eighth day after birth. Also known as Brit Milah..
In Judaism, the infant is baptised before it has a faith of its own to symbolise that it belongs to the Jewish people. In the same way, Christian baptism of an infant occurs before the infant has a faith of its own and shows that it belongs to the Church of Christ.
Both circumcision and infant baptism look forward to a time when the child will choose to believe for themselves. Therefore, infant baptism is seen to replace circumcision in the Christian Church.