I think that most people who say they are atheists and agnostics say this because they want to live their lives the way it seems right for them. They don't want to be morally responsible to anyone else, especially God. So they take an atheistic or agnostic stance because it helps them avoid being morally responsible for the way they live and what they do.
So, anything goes, they can be a lying, thieving, blaspheming, fornicating adulterer at heart, and not have to account for what they do, because they believe that when they fall off the perch and the lights go out that's it. Oblivion and nothing more.
But what if there is an afterlife, and there is a judgment where we all have to stand before God and answer for how we live and what we do? What would we do if we are found guilty as charged and have to face a sort of an eternal prison without parole? I think that every person, even though they are atheist or agnostic, know it deep down in their conscience that there could be something out there in eternity.
Ask an atheist - it is right to rape three women and cut their throats? Would he think that is right? What if it was his sister who was one of those women? How would he feel about that? I know that he would exclaim, "That is so wrong!" But what makes it wrong? There is no standard of morality and no judgment so who cares?
But the reality is that something within him cries out about the wrongness of such actions, because he has a conscience, whether he listens to it or not. This conscience is God's warning device, like a smoke alarm, and it goes off when something is wrong. This means if there is a judgment, God will tell the atheist, "Why did you not listen to your own conscience?" and he will have no answer. He will have to acknowledge that he is guilty and that all he can expect is eternity in "prison" which Christians call hell, without the possibility of parole.