Acts 2:38 NKJV    38 Then Peter said to them, Repent, and let every one of you be baptized in the name of Jesus Christ for the remission of sins; and you shall receive the gift of the Holy Spirit.

You have signed up to declare your faith in Jesus Christ publicly. Being lowered into the water symbolizes dying to an old life, and rising from the water is being resurrected as a new creation in Jesus Christ.