Abrahamic religions often hold that there is some sort of reward or punishment in the afterlife. These are, of course, heaven and hell. They hold that if you do or don’t do certain things, you’ll go to hell and burn in sulphurous hellfire for eternity. Similarly, they hold that if you do do or do not do certain things different things, you’ll be in paradise forever.
In a similar vein, other religions believe in karma. They hold that if you do or don’t do certain things, you’ll come back as pond scum or the like. Similarly, if you do do or don’t do certain different things, you’ll stop reincarnating and achieve Nirvana.
Recall how many of those religions emphasize the desirability of in–conversion. They often try to get you to convert by emphasizing what will happen to you if you don’t convert; that is, bad things will happen to you. To you. Notice who the emphasis is on. You. This is therefore immoral.
Because it’s selfish, that’s why. Belief in afterlife punishment or reward causes people to be motivated by a selfish desire to avoid something bad happening to them (such as burning in hellfire forever) and to get something good to happen to them (such as living in heaven). It causes people to directly consider what happens to them. Although it might indirectly influence someone’s interactions with others (for example, you’ll burn in hell if you aren’t nice to other people) it still ultimately motivates based on what happens to the actor. A truly good person would be good purely for the sake of being good; they would need no other motivation. When asked, “What is in it for you?” they would reply “Nothing, but I’m being moral anyway.”
I have thought of a few possible objections to my conclusions. Hence, I’ve taken the liberty of preemptively debunking them. And that is after the jump.