For starters:
1. They would have to consider that they may be morally accountable to something greater than themselves
2. They would have to consider what happens to them upon death, and the possibility of them being judged for their actions in this life (for example, if I was a murderer or a rapist, I would probably be hoping that there is no God, and that I cease to exist upon death)
1. There are approximately 4,000 to 5,000 recognized religions in the world.
2. Each one of the religions, insist that "theirs" is the one true religion and the others are false.
3. It is said that Hinduism, is the first "known" religion. It is a polytheistic religion with 1.2 billion (that's with a "B") worshippers. They too claim theirs is the true religion.
4. Religions that "may" actually be older than Hinduism, are the tribal Animist or world of spirits in things like trees, rocks, et cetera. Some still hold to them.
5. One thing they all hold in common, are the vast, vast majority of them were created in eras in which the world was filled with primitive peoples harboring copious amounts of superstitious ideas and had little to no science in their understanding of the world and universe.
6. As intelligence and science progressed, many things considered the works of invisible deities and that lights in the night sky were departed human souls, were revealed to have actual scientific explanations.
7. The primitive peoples feared death and didn't want life to end, concocting stories of afterlives to appease those fears, or in some cases, reincarnation to come back again as people or other living things.
8. Not knowing about weather pattern changes, some turned to sacrificing living things, including people to appease the imaginary deities
9. The Abrahamic religions are just a broad group of three religions that needed to create something for the people to fear, even after death, in an effort to control them. Actual events of their eras, combined with moralistic tales, written down and preached to the superstitious people ensured relatively cohesive rule abiding societies.
As to Atheists, so far, all we've seen are most often explained by science. That doesn't mean we don't have what you would call morals. I like most others, don't like our material things to be stolen, thus, it also means, we don't believe others should have their things stolen. We don't want to be beaten or murdered, so we believe that others shouldn't be beaten or murdered. We believe that laws are important to maintain cohesive societies and reduce endangering the populace.
We honor/respect our father and mother, IF they are not complete douchebags. Most of us believe that the truth is important, not because of some so-called holy book, but because we would like to hear honesty from those that we meet. We do excuse what they call little-white lies, so as to not hurt someone's feelings.
As to adultery, if you have sworn to be faithful to another person, you should keep your word. However, even the religious crowd breaks their own rule on that.
No day to us is holy. Period.