Why are Christians baptized?
This is a good question. Let me make a few points on the Christian practice of baptism.
First, Christian undergo baptism, because it is commanded in the New Testament. For example, in Matthew 28:19, Jesus says that the disciples should go around the world to teach people about him and baptize people.
"Therefore go and make disciples of all nations,baptizing them in the name of the Father and of the Son and of the Holy Spirit..."
Second, when we approach Paul's theology, Paul states that baptism represents the initiation of the Christian to into a new community. Within this framework (Romans 6), baptism represents death and resurrection. As one goes into the water, he or she experiences death to sin and as the person emerges from the water, he or she experiences new birth. In other words, the person is new in Christ. This is why Paul states the following in 2 Corinthians 5:17:
"Therefore, if anyone is in Christ, he is a new creation, the old has gone and new is present."
Finally, baptism is also given to infants in many churches, because there is a belief that the faith of parents is passed down to their children.