When a person is baptized, that person then becomes one with Jesus Christ. This is depicted in Romans chapter 6 - "Know ye not, that so many of us as were baptized in Jesus Christ were baptized into his death?"
Therefore if anyone be in Christ he is a new creature: old things are passed away and all things are become new.
When one is baptized ones old self is buried, and a new self is resurrected. At this point sin no longer has power over the baptized individual leading to death. For the person who is dead is freed from the lust of sin.
Having said this, the bible warns that although the person is dead to sin, returning to a life of sin can also return one to be the servant of sin, which leads to death.
Some believe that the baptism is only a symbolic means of proving to the world that one is a Christian, but this is far from the truth. Jesus said unto Nicodemus who was a Jewish leader that unless one is born again in the form of baptism, then one can not see the kingdom of heaven. Therefore it is beyond question that believers be baptized.
The bible says that those who believe must repent of their sins in concert with the baptism, to be saved.