The existence of a God automatically creates an ultimate standard of right and wrong because whatever God (often through His priestly class) declares to be right or wrong, is so. Even things which any normal human being would instinctively consider abhorrent, such a mutilating the genitals of a child, become moral if they are commanded by God.
This is very attractive to people who aren't very comfortable working things out (thinking) for themselves, which covers most theists. They don't have to put any real consideration into the morality of their behaviour; it suffices that they do as they're told by the people they consider moral authorities.
If the Pope tells them to campaign against condom use in AIDS-stricken regions, or the God-anointed Tsar tells them to ghettoize and starve Jews, or the local parson tells them to chain and drag a homosexual behind their pickup truck, then they'll obey, because obedience to the Word Of God is the greatest virtue, as is demonstrated time and again in the holy books.
There is no better exemplar than Abraham, who is accounted most holy because of his unquestioning willingness to kill his beloved child because the voice in his head told him to, prior to changing its mind at the last minute. (Pity it didn't change its mind before all the other childslayings and genocides, but ours not to reason why, eh?)
That voice in his head was actually the voice of the LORD GOD ALMIGHTY; we know this because Abraham did rather well for himself afterward, and was even promised that his descendants would be looked after also, as long as they obeyed a few simple rules mostly about diet and genital mutilation. Had Abraham or his descendants done badly at any point, we'd have known that he was just a nutcase after all.
Do you see how superior this system is, compared to that of the rationalist? You never need spend any mental effort wondering what is the right thing to do, because a kindly priest or anointed king is always there to inform you. There's even an automatic feedback system, because if it turn out that they're false priests or kings then it will be obvious when God foils their plans.
The poor atheist, on the other hand, is beset with uncertainty, having to constantly think about what the best course of action is, with no guarantee from authority that they are right; the atheist has to actually consider what is best for everyone, has to see the situation from everyone's point of view, not just their/the priests'/God's preferences.
It's exhausting, really. Small wonder that theism tends to attract the morally as well as the intellectually lazy. Who wants to go to all that trouble of gathering evidence and thinking and testing the validity of your beliefs, when you can just outource it all to God and/or His representatives?
And after all, how far wrong could we go in just following orders we've been assured are lawful?