Because IBelieve
Baptism is an outward picture of an inward commitment to follow Jesus in obedience. Similar to a marriage ceremony where the bride and groom publicly profess their love for and dedication to one another before God, their friends, and family, when we get baptized we publicly profess our love for and dedication to follow Jesus in obedience.
Water Baptism preaches the gospel to the eyes. When we see someone standing in the water, they are showing that they were dead in their sins, but when they heard the gospel it changed them. When we see them being submerged in the water, they are burying their old self. Finally when they are raised from the water, they identify with Jesus and are raised to new life in Him.

Who Should Be Baptized?
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Baptism FAQ
For by grace you have been saved through faith. And this is not your own doing; it is the gift of God, not a result of works, so that no one may boast.
Ephesians 2:8-9 (ESV)