Many theists believe it is clear-cut. Humans can only have opinions about morality, and no one’s opinion is any more valid than anyone else’s. This leads them to the conclusion that an objective source of morality must stand apart from, and above, humans. That source, they say, is God. Since atheists, reject God, atheists can have no basis for morality.
This is really two separate arguments: (1) that God is the source of objective morality and humans can learn morality from God and (2) that humans on their own have no way to know what is moral and what is not.
Can atheists be moral? - Atheist Alliance International
Morals are very personal. I am an atheist, have never been religious, and I have my own very strong set of morals.
However, in my experience, I've noticed that a huge proportion of people, many in this very forum, who are disillusioned Christians, rather than atheists from birth such as my siblings and myself, do not seem to have any morals whatsoever. Because their original morals taught to them as children by their family were based squarely on their religion, when they lose their religious faith, their morals tend to go out the window with it.
They end up being extraordinarily immoral piece of shit motherfuckers when they get older.
AKA Democrats.