A fully enlightened person took his atheism to the absolute degree. Atheists don’t believe in God, souls, spirituality, or life after death. So they are already ahead of the curve, as far as enlightenment goes. But they still do believe a lot of other stuff, and psychologically, they are not that much different from a person who believes in God. Somewhat more mature (there is no Santa Claus after all), but not that much different. Every form of suffering comes from an irrational belief, and beliefs that bring most suffering are those about oneself, not Gods. A religious mind is just more prone to fantasies, which is why religious people are more aggressive, scared (for example, of sin, sex and punishment), and close-minded than atheists, but compared to full enlightenment, the difference is subtle. Visible, yes, but subtle.