Friedrich Nietzsche was a 19th century atheist who understood the implications of his own atheism. If God doesn’t exist, he says, then there are no transcendent morals, values, or truths. Individuals have to create their own. This means that the strongest man, by exercising his “will to power,” will dominate by imposing his own truth and values on everyone else. Nietzsche understood that without God, there’s no basis for objective morality.
Yet more than a hundred years after his death, people still haven’t been able to put this kind of moral relativism into practice. Even people who don’t believe God are still driven by their moral intuitions. No sane person is a moral relativist in practice.