A person makes a decision to get right with God in view of his experience of the enslaving power of sin and in view of the promises in the Bible about the benefits of following Christ. The Red Sea Crossing is a baptism of repentance, like John the Baptist offered. This was in preparation for following Christ. Many may hear the gospel, but not get it. They hear, but they don't fully understand the essence of the message. They may get water baptized and make a verbal confession in all sincerity, but in their heart not yet really received the message. And it may take years in a church environment before they receive the revelation. They may become very active Christians, not realizing that they haven't come to understand the message, and not having been born of God yet. Paul ran into some of these brethren.