Can an atheist act in moral and ethical ways? Certainly, he can. All humans still retain the image of God upon them, even after the fall of Adam and Eve into sin. The image of God was effaced at the fall, but it was not erased, and so man still understands right and wrong no matter how many try to say otherwise. Even atheists react to this inherent knowledge of right and wrong, some even to the extent of living exemplary lives.
C.S. Lewis put it this way: C.S. Lewis described this well. He noted that if a man sees another in danger, the first instinct is to rush to help (altruism). But a second internal voice intervenes and says, “No, don’t endanger yourself,” which is in keeping with self-preservation. But then a third internal voice says, “No, you ought to help.” Where does that third voice come from, asks Lewis? This is what is referred to as the “ought-ness” of life. Morality is what people do, but ethics describe what people ought to do. And yes, people know what they ought to do, but that doesn’t mean that they always act according to that knowledge.
The difference between the atheist and the Christian in this sense is that the atheist may act ethically for certain reasons (e.g., not wanting to go to jail, it disrupts social order, it makes them look good to others, etc.), but he has no ultimate reason for acting ethically because there is no ultimate moral authority that exists over each sphere of his life. Without this ultimate authority, each atheist defines morality on his own terms, although his morality is influenced by the remnants of morality from the image of God within, along with the strictures and constraints of the culture and society in which the atheist exists.
The Christian, on the other hand, acts morally out of the knowledge of the moral law given by God in His Word and a love for the Law-giver Himself. In addition, that knowledge is continually increased and personalized by the indwelling Spirit of God, whose task it is to bring the Christian “into all truth” (John 16:13). From within believers, He directs, guides, comforts, and influences us, as well as producing in us the fruit of the Spirit (Galatians 5:22-23). To the atheist who is without the Spirit, God’s truth is “foolishness,” because it is “spiritually discerned” (1 Corinthians 2:14), and the only fruit of righteousness is self-righteousness, not the righteousness of Christ.
When confronted with a situation that demands both the Christian and the atheist to make moral choices, a situation in which societal constraints are removed, the reaction of each will be vastly different. If a society deems it morally acceptable to kill unborn babies, for instance, the atheist sees no reason to oppose the practice. His own “moral law” even tells him it’s the compassionate thing to do in cases where the child is the result of rape or incest. The Christian, however, knows abortion is wrong because his moral choices are built upon the moral Law-giver who has declared all human life to be sacred because it is created in the image of God. The Law-giver has proclaimed, “You shall not murder” (Exodus 20:13) and, for the Christian, there’s the end of it.
So can an atheist act ethically? Certainly, but he has no ultimate reason to do so and no ultimate authority to look to in order to ensure his line is indeed straight and unbendable.