To be a Christian requires us to live a distinctive life that is different. It requires a commitment beginning with our conscious turning from known sinfulness in our lives such as selfishness, lying, hatred, bitterness, a lack of faithfulness in our relationships and being determined to put Jesus first in our lives. Christians are called to love God with all out heart and mind and to love our neighbours as ourselves in a lifestyle that reflects the character of Jesus.
The decision to do this is sometimes called Conversion or turning to Christ. It's a decision that will affect our beliefs and our behaviour.
Baptism identifies our decision to be a Christian once and for all. It's a bit like getting married - a public confession of your loyalty to Jesus Christ; a lifelong commitment and a time to celebrate. In baptism also God cleans us from the inside out and gives a fresh new start with the beginnings of our new charter which takes a lifetime to further develop!
So baptism indicates our commitment to Jesus; it also indicates our commitment to Jesus' family, the Church. Indeed baptism is a baptism into the family of Jesus - the family of all Christian believers throughout the world. In the New Testament, baptism indentified who the true believers were.
Some of us are Jews, some are Gentiles, some are slaves, and some are free. But we have all been baptized into one body by one Spirit, and we all share the same Spirit.
(1 Corinthians 12 v 13)
When a person is baptised (normally during a Sunday morning service) the candidate makes a statement to show they are repenting of their sin and choosing to follow Jesus Christ, acknowledging that Christ died in their place and their determination to live for him thereon after.