Except for those pertaining about God, an atheist can do moral acts. He can even follow the commandments from the 4th through the 10th.
The difference between an atheist and a Christian is the “works” of an atheist originated from him/her; from “flesh-and-blood”. On the other hand, the works of a Christian emanates from the Spirit of God, given by grace and through the death and resurrection of Jesus. Whatever “works” any Christian does that come from his own “flesh-and-blood” are not legitimate nor acknowledged at the highest level that God requires either. Notice the principle that it is God, not anyone else, who decides and chooses what is best and acceptable to Him.
Heb.10:1-6 “For the law having a shadow of good things to come, and not the very image of the things, can never with those sacrifices which they offered year by year continually make the comers thereunto perfect. For then would they not have ceased to be offered? Because that the worshippers once purged should have had no more conscience of sins. But in those sacrifices there is a remembrance again made of sins every year. For it is not possible that the blood of bulls and of goats should take away sins. Wherefore when he cometh into the world, he saith, Sacrifice and offering thou wouldest not, but a body hast thou prepared me, In burnt offerings and sacrifices for sin thou hast had no pleasure.” (Remember, all “temple sacrifices”, were not actually “acceptable” to God because he requires a “living sacrifice” (Jesus). See also Rom 12:1)
Gen 4:3-5 “And in process of time it came to pass, that Cain brought of the fruit of the ground an offering unto the LORD. And Abel, he also brought of the firstlings of his flock and of the fat thereof. And the LORD had respect (or regard) unto Abel and to his offering: But unto Cain and to his offering he had not respect.” (God chooses what he wanted and prefers a lamb, firstborn.) Similarly, what God wants and requires are “works” of man that emanates, not from man and on his own, but from the Spirit of God in us and given to us as a gift, by grace. The “fruits of the Spirit” (Gal.5:22-23) have to manifest in us. In fact, this Spirit of God is so precious for us to receive, that it took the faith (of) Jesus (in) the Father that what was promised to Jesus, if he divest his powers as Logos/YHVH ELOHIM/Creator and dies as a man, would be fulfilled at his resurrection (Jn.16:7; 14:16,25,26). It took the “faith and works” (of) Jesus, his death as a man and his resurrection as a living “spirit-being”, that we received this “gift of Holy Spirit”. The preceding is a very important principle not yet emphasized this way by traditional Christianity.
Good morals are the fruit of being a Christian. Without knowing and loving God the good moral acts of an atheist is not a moral act it is just an act that in whatever way serves the atheist. If the atheist did what a Christian would consider moral I am of the impression that this act is self fulfilling to the atheist and not moral – it serves only the atheist who now has an idol to worship – himself.
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: if a man sees another in danger, the first instinct is to rush to help (altruism). But a second voice intervenes and says, “No, don’t endanger yourself,” which is in keeping with self-preservation. But then a third voice comes into play and 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 un-bendable.