Jesus commissioned His followers to go and make disciples, "baptizing them in the name of the Father and the Son and the Holy Spirit" (Matt. 28:19). As the early church spread the gospel message, baptism would follow a new believer's response of faith. It publicly signified that the individual was now a follower of Jesus
Metaphors often communicate on a level that words cannot. Baptism is a powerful picture of our salvation experience. Through this act, we proclaim the good news that Jesus died for our sins, was buried, and rose again--and testify that we've welcomed His transforming power into our life.
The Greek word for "baptize" in Scripture is the same term used to describe a cloth dipped in dye--it refers to total change. So by being plunged into the water, we declare that we're choosing to die to our old way of life and are uniting with Christ. Our sin is buried with Him, and its power is conquered through His atoning death on the cross (Rom. 6:14). When we're raised up out of the water, we affirm His resurrection. Baptism is a symbolic way of expressing that just as the Lord conquered death and rose again, we are spiritually resurrected from death into new life. We are "born again" and irrevocably transformed through the power of His Holy Spirit.
In the Bible, the word "believe" isn't a conceptual word describing intellectual agreement alone. It is a word of action. Our belief should never be hidden like a light placed under a bowl (Luke 11:33)--when unbelieving family and friends look at our lives, they need to see the gospel in action.