Why do we baptize?
"Or do you not know that as many of us as were baptized into Christ Jesus were baptized into His death?  Therefore we were buried with Him through baptism into death, that just as Christ was raised from the dead by the glory of the Father, even s we also should walk in newness of life. For if we have been united together in the likeness of His death, certainly we also shall be in the likeness of His resurrection." - Romans 6:3-5

We baptize in accordance to the Word of God, which tells us all Christians who have given their lives to the Lord Jesus Christ are expected to take part in this ordinance of conversion.  It is a sign of surrendering, dying, and rising up with Christ.  It is for those who believe and are willing to serve and walk with the Lord Jesus.  It is for Christians who have faith and have repented of their old ways.