If life ends at the grave, then it makes no difference whether one has lived as a Stalin or as a saint. Since one’s destiny is ultimately unrelated to one’s behavior, you may as well just live as you please. As Dostoyevsky put it: “If there is no immortality then all things are permitted.” On this basis, a writer like Ayn Rand is absolutely correct to praise the virtues of selfishness. Live totally for self; no one holds you accountable! Indeed, it would be foolish to do anything else, for life is too short to jeopardize it by acting out of anything but pure self-interest. Sacrifice for another person would be stupid. Kai Nielsen, an atheist philosopher who attempts to defend the viability of ethics without God, in the end admits:

We have not been able to show that reason requires the moral point of view, or that all really rational persons, unhoodwinked by myth or ideology, need not be individual egoists or classical amoralists. Reason doesn’t decide here. The picture I have painted for you is not a pleasant one. Reflection on it depresses me. . . . Pure practical reason, even with a good knowledge of the facts, will not take you to morality.[7]

But the problem becomes even worse. For, regardless of immortality, if there is no God, then there can be no objective standards of right and wrong. All we are confronted with is, in Jean-Paul Sartre’s words, the bare, valueless fact of existence. Moral values are either just expressions of personal taste or the by-products of socio-biological evolution and conditioning. In the words of one humanist philosopher, “The moral principles that govern our behavior are rooted in habit and custom, feeling and fashion.”[8] In a world without God, who is to say which values are right and which are wrong? Who is to judge that the values of Adolf Hitler are inferior to those of a saint? The concept of morality loses all meaning in a universe without God. As one contemporary atheistic ethicist points out, “to say that something is wrong because ... it is forbidden by God, is ... perfectly understandable to anyone who believes in a law-giving God. But to say that something is wrong ... even though no God exists to forbid it, is not understandable....” “The concept of moral obligation [is] unintelligible apart from the idea of God. The words remain but their meaning is gone.”[9] In a world without God, there can be no objective right and wrong, only our culturally and personally relative, subjective judgments. This means that it is impossible to condemn war, oppression, or crime as evil. Nor can one praise brotherhood, equality, and love as good. For in a universe without God, good and evil do not exist—there is only the bare valueless fact of existence, and there is no one to say you are right and I am wrong.

Follow this link to read the whole chapter
The Absurdity of Life Without God - 'Reasonable Faith', Chapter Two