So ultimately, for the free adult, without God, there is no reason to be moral. We can limit our own behavior because we feel like it, or because from a utilitarian perspective it makes sense at a particular time in a particular circumstance, but choosing to accept that there are moral imperatives is irrational unless there is an authority (God) that makes them imperative on all of us.
Cyberstone we exist as a species to a great extent because we have a structured society, and a stable society accommodates progress. Co habitation requires some form of moral value system, but societal rules run against one of our primary survival mandates, the one that should and does say to us "take every advantage". Society works because we have gone from wolves to sheep, we are a society of predators who have a propensity to covet and prey on our neighbors, so rules and enforcement are needed to keep us from turning to the wolves we are.
All major religions have a "Ten Commandments" type moral directive laying out the rules of engagement. They are all the same, no lying, no stealing, no killing, no coveting, no sleeping with the neighbors partner. Five common sense rules that stop a society tearing itself apart.
The question of why we adhere to these mandates against our will at times is imo a meld of evolutionary and generationally learnt realities. We have a consciousness that allows us to understand time and death and that understanding is the mother of religion. We contemplated why are we here if we are not here forever, that is why all religions give the faithful an eternal life after death, if you adhere to the teachings, and the moral ones are second only to worship one god. So the moral compass engendered by religions is certainly a factor in behavior.
Generationally learnt is the holistic view of the society on honesty. It is an expectation that except for the percentage of thieves and liars that all societies have carried you can leave an expensive piece of jewelry with a craftsman and they will not deny that you ever gave it to them when you return for it. It is simplistic but true, society works at the most basic level on trust, we have a conscious understanding that complete moral anarchy destroys the society and our species chance of survival.
So a real or invented god coupled with a conscious understanding that social stability promotes the species sees most of us toeing the line, to varying extents.