This is a very old Christian tradition. The best ancient example is the Emperor Constantine, who made Christianity the official religion of the Roman Empire, amidst great cruelty, treachery etc., but who wasn't baptised himself until he was on his deathbed. All his sins got washed away. It was a common practice back then.

The idea is that forgiveness is possible through Christ, that if you're truly sorry for the bad things you've done, you will be forgiven. Free pass. Clean slate. Ally ally outs in free.

All whole lot of wickedness has been accomplished over the past couple of millenia by persons who believed in heaven and especially hell but knew that they could get away with it as long as they didn't die inconveniently young.

Religious American right-wingers aren't all that original, you know.