Because the Bible states that it is;
The New Testament mentions baptism 51 times. Every conversion to Christ recorded in the book of Acts ended in a water baptism. And every New Testament writer considered baptism an essential part of a believer’s response and connection
to Jesus. The Apostle Paul wrote that the gospel, which saves us, is the death, burial, and resurrection of Christ on our behalf - to atone for our sins ( Corinthians. 15:1-4 ). He also wrote that water baptism is a
faith-inspired re-enactment of that gospel. It’s a symbolic death, reflected by burial in water (instead of dirt) and a symbolic resurrection acted out by rising from the water ( Romans 6:1-7 ).
Christ considered it so important that He Himself was baptized. He included this teaching in his marching orders as He sent his apostles out to make more disciples ( Mark 16:15-16 ).He said to them, “Go into all the
world and preach the gospel to all creation. Whoever believes and is baptized will be saved, but whoever does not believe will be condemned.