I don’t believe in once saved, always saved. My understanding of grace is that there is no sin that you cannot repent from and be forgiven for, but you still must turn from sin.
I always held the notion that if you accepted Jesus, yet continued to sin unrepentantly, you really never accepted Him in the first place.
How about a fast-acting suicide?
If one was capable of "turning from sin" then Christ died for no good reason. I go by the belief that the only unpardonable sin is to blaspheme the Holy Spirit and I also believe that if one has been saved, they will be unable to do so just as Satan (and his minions) are unable to proclaim Jesus as Lord of, and Savior to, all.
Perhaps you recall Paul's lament about how, despite his faith and knowledge, he constantly found himself "doing that which he would not do, and not doing that which he would do."
Those that claim to totally move away from sin are either fooling themselves or acting as the hypocrites that Jesus claimed the Pharisees were with their publicly pious stage shows.
Paul talks about believers falling away. He even speaks of the inability for Christ to be crucified again.
There will be no atheists in Heaven, even if they did walk the aisle as a teen.