Baptism is not a prerequisite for salvation but rather an outward display of Christian faith. It is a ritual that inaugurates one into the public body of Christian believers. If the thief had gotten down from the cross, he would have surely been baptized. He didn't, obviously. Constantine is the opposite. He had his whole life to make an official public display of faith through Baptism and refused until he thought that he needed to do so at the end of his life.