Why must a believer in Jesus be baptized in water?

Now, even though the Apostle Matthew begins his written Gospel of Jesus’ ministry by first telling us about Jesus’ water baptism, the Apostle Matthew ends his written Gospel by telling us about our water baptism. Sometime after Jesus died on the cross and rose from the dead, but before He ascended into Heaven, He said this to His Apostles in Matthew 28:18-20:

18 Then Jesus came to them and said, “All power in heaven and on earth is given to me. 19 So go and make followers of all people in the world. Baptize them in the name of the Father and the Son and the Holy Spirit. 20 Teach them to obey everything that I have taught you, and I will be with you always, even until the end of this age.” (NCV)

From this passage, we see that those who become followers of Jesus are to be baptized in water. We also read throughout the Book of Acts that those who believed in Jesus’ death on the cross and bodily resurrection were baptized in water. So, why must a believer and follower of Jesus get baptized in water? Based on the Matthew 28:18-20 passage, if you are going to be a believer and follower of Jesus, your first step in following Him is to obey His command to be baptized by following Him into the waters of baptism and be baptized just as He was baptized.