In a one-off encounter between different species, selfishness pays (e.g. lions and cheetahs), but not in a social atmostphere of one species (e.g. amongst a troop of chimpanzees). The religious would ignorantly counter by saying “You wouldn’t steal from a family member but you’d steal from a stranger!” which is a load of crap. Socialization is more than family, it’s communities, cities and countries. Even as a tourist in a foreign place, people tend to act ethically because we are concerned about what others think of us.
Atheists want to live in a civilized society, so we try to make it one. By voluntarily working for the benefit of others as well as ourselves, we gain a social structure, both community and relationships. The religious are also motivated by selfishness, but they are more concerned with the myth of “going to hell” and having a spit inserted up their nethers and out through their mouths. They don’t see the need to be civil or cooperative except where there is personal benefit, not because it is “the right thing to do”.
The two may sound similar, but atheists don’t believe the lie of “I can pray and be forgiven!” Forgiveness from a god is an excuse for hypocritical behaviour – wanting the protections of society but none of its obligations.
That pretty well explains why I don’t think belief in heaven and hell makes a person moral. It’s a cheat to get similar results, but with many flaws.