If they are good, it is because they reasoned it out that it feels better to be a good person and not intentionally inflict harm on others. Not because they bribe “God” with promises and sacrifices to gain favor and possibly go to heaven. An atheist might even think the standardized view of heaven is boring. Besides, all this heaven and hell business is putting things off into the future, while the enlightened life is living totally in the Now.
It might even be argued that organized religions are a deterrent to enlightenment, especially if they discourage followers from arriving at their own conclusions and encourage them to rely on the leadership to tell them what’s right, what’s wrong, and why they are going to hell after they die, according to some book created by people long ago.
Atheists have allowed themselves to be individual thinkers and can achieve deep, inner peace thereby. Besides, jails are full of violent criminals tattooed with all kinds of religious symbols who swear they are whatever religion. And, of course, the greatest brutalities in the world have been perpetrated by religions for centuries (including human sacrifice, torture during the Inquisition, “holy wars,” and other events too numerous to mention that are going on right now).
Enlightenment requires deep, inner mining of our Souls. It is arriving at our own world view based on inner experience and contemplation. It is the stilling of the ego-mind and allowing other faculties to come forward, which bring an underlying peace, regardless of outer circumstances. Whether we belong to some group or another, has nothing to do with enlightenment. It has nothing to do with being a good person. For if we are good only because we fear the wrath of God, we are just covering all our bases out of fear. Living a fearful life and worrying about the future is not enlightenment.