First of all, their people who believe in Jesus Christ but have never been baptized with water, to symbolize your commitment to him to the entire world. There are people who come to Christ and yet have never been baptized with water. Does that mean they are not saved? No, only
through Jesus Christ and believing in His name and accepting what He did for you, on the cross can one be saved. So, water baptism is not something needed to be saved, but does help understand what we do when we come to Christ. When I came to Christ in 2002, I was never baptized with water. Did that mean I was not saved? No, it means I never went through that. As for God letting something slip, this is Jesus we are talking about and he is God and God will not let anything slip. He knew that the heart of the thief on the cross was genuine, hungered for him, and realized that he was God and only through Him could he be saved. Think about some Christians who come to faith in persecuted countries like the Middle East or other places, many come to Christ even though they could be killed. Some before they can be baptized are slaughtered like animals by those who hate God and reject Jesus. Does it mean those Christians were not saved? No, it means they did not have time to be baptized with water. They already were baptized with the Holy Spirit but not with water. God bless and I hope that helps. Those are my two cents.