The Bible teaches the baptism of believers (see Mark 16:16, Acts 8:36-37). And in the early centuries, Christian baptism was by immersion. The concept of infant baptism is not specifically found in the Bible.
The early church began baptizing infants because of the idea that children are born in sin and need immediate baptism to ensure their salvation. At the time of the Reformation, though, Martin Luther rediscovered the biblical truth that spiritual regeneration and justification are by faith. A baby cannot exercise faith. Nevertheless, because so many of those who became his followers had already been baptized as infants in the Roman Catholic Church, Luther did not require them to be baptized again. In fact, he continued to baptize children of believers.
When John Calvin came on the scene in Geneva, he taught what is known as "covenant theology." According to covenant theology, if the parents have entered a covenant relationship with God, then their children are part of the covenant, too, and are proper candidates for baptism as infants.
In churches that practice infant baptism, there is usually some type of confirmation rite in which a child of 10, 11, or 12 reaffirms the parental faith expressed at his or her baptism.
Some Reformation theologians strongly disagreed with infant baptism. They maintained that people should be baptized only as believers, because baptism is a symbol of the death, burial, and resurrection of Jesus (see Romans 6:4, Colossians 2:12). Baptism to them was the end of the old life and the beginning of a new life in Christ. Since a baby does not have an old life to deal with, they maintained, if someone has been baptized as an infant, he need to be baptized again as an adult believer. These people were called "Anabaptists," which means "rebaptizers."
Today, there is still disagreement about the proper form of baptism. But, more and more, Methodist, Presbyterian, and some other churches that have historically practiced infant baptism are looking anew at both the qualification of a candidate for baptism and the mode of baptism. A number of people, even though they may have been baptized as infants, are being rebaptized by immersion as adult believers.