Here's a guy who gives his life to Christ, The Holy Spirit has been prodding and encouraging him with all kinds of understanding. He hits his knees and says the sinners prayer worded just like Franklin Graham told him to. There can be no doubt this guy is saved! By every denominational standard he is heaven bound.
The Holy Spirit keeps talking to him. You need to to get baptized . You need to show that outward expression of that inward feeling you have of already being saved.
The preacher puts him down in the baptistry and when he pulls him up out of the water, he pulls a gun out and shoots the preacher in the face and kills him dead right on the spot.
No worries. Once saved, always saved.