Baptism is a central component of the Great Commission because baptism marks us out as followers of Christ and assures us that by faith we have been forgiven by God. There's a lot that can be said about the meaning of baptism. It signifies our dying and rising with Christ. It signifies our sprinkling with the blood of Christ. It signifies our union with Christ. All of this is true and worth our contemplation. But at the most basic level, the water of baptism reminds us that our sins have been washed away. All our stains have been wiped clean by the sin-scrubbing detergent of God's grace.
DeYoung, Kevin. The Good News We Almost Forgot: Rediscovering the Gospel in a 16th Century Catechism. Chicago, IL: Moody, 2010. Print. 128 - Emphasis mine.