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
I'm very late in this dance, but I am atheist. But I am very moral. I need no reason to be moral, I just am.
Evolutionist theory would say that every species should be moral, unless they lay millions of eggs. Morality is being nice to your own species so you can survive.
A species that kills their own, unless they lay thousands of eggs, will not survive for the long run.
Being "good" is an evolutionary trait, not a religious one.
There are many examples of animals that aren't "moral", but those are one-off and rare, like humans. Most animals respect this "moral code". Because that is the basis of surviving.