Believers' Baptism is a step taken after a commitment to follow Jesus as a public witness of that step of faith.
Normally baptism takes place by full immersion and symbolises being united with Christ in his death and resurrection.
Having been buried with him in baptism, in which you were also raised with him through your failth in the working of God, who raised him from the dead (Colossians 2:12)
There are many reasons why believers choose to be baptised:
Jesus was baptised by John the Baptist Matthew 3:13-17; Mark 1:9-11; Luke 3:21-22. Therefore, to be baptised is to be obedient to the path set out by Jesus.
In the Great Commission, Jesus says, Therefore go and make disciples of all nations, baptising them in the name of the Father and of the Son and of the Holy Spirit... Matthew 28:18-20
The early church practised it: Peter replied, 'Repent and be baptised...' Acts 2:38-39
There are examples of baptism throughout the book of Acts which suggests believer's baptism was an integral part of early church life. But it's not just about forgiveness of sins and dying to an old way of life. Baptism symbolises our response to how God has saved us through Jesus.
18 For Christ also suffered once for sins, the righteous for the unrighteous, to bring you to God. He was put to death in the body but made alive in the Spirit. 19 After being made alive, he went and made proclamation to the imprisoned spirits - 20 to those who were disobedient long ago when God waited patiently in the days of Noah while the ark was being built. In it only a few people, eight in all, were saved through water, 21 and this water symbolises baptism that now saves you also - not the removal of dirt from the body but the pledge of a clear conscience towards God. It saves you by the resurrection of Jesus Christ, 22 who has gone into heaven and is at God's right hand - with angels, authorities and powers in submission to him.
1 Peter 3:18-22