Except for the bit where if you've outright left said religious group, it's likely because you don't believe in their teachings anymore anyways, so the threat is empty air to you. The only people who put any stock in its validity are the ones who it doesn't apply to, the faithful within that particular group. It's the religious equivalent of 'you want to leave? fine, but you're never allowed back in'; making sure people who want to leave really want to leave, because a faith doesn't want people who are on the fence about their beliefs. Every major religion in history follows this template to some degree - the stick of 'if you're not with us, you are doomed' to go with the carrot of 'if you are with us, you're saved'.