St. Paul, referring to the Pagan Greeks said this:
"For it is not the hearers of the law who are righteous before God, but the doers of the law who will be justified. For when Gentiles, who do not have the law, by nature do what the law requires, they are a law unto themselves, even though they do not have the law. They show that the work of the law is written on their hearts, while their conscience also bears witness, and their conflicting thoughts accuse or even excuse them on that day when, according to my gospel, God judges the secrets of men by Christ Jesus." - Romans 2:12-16
The broader context is, ultimately, "All have sinned and fall short" (Romans 3:23), but the Apostle here is condemning those who believe themselves righteous because they received the Law. As though knowing the Law, but failing to do the Law, somehow makes one better, or more righteous, in the sight of God; it is not merely knowing what is written in the Law that matters, it is doing what the Law says that matters. So when a Pagan does what is right and in accordance with the Law, it is better and more noble than when someone who knows what the Law says but fails to do it anyway, "While you preach against stealing, do you steal?"
So if an atheist does what is in accordance to the revealed goodness and justice of God, but the Christian does not, then what does that say? When we stand on that Great Day before the Throne of Judgment and all our secrets are exposed before the light, which will prevail? Our muttering "Lord! Lord!" or actually doing the will of our Father in heaven (Matthew 7:21-23).
If we claim to fear God, but instead live emboldened by our sin, then we receive a double portion of condemnation. I do not expect an atheist to follow Jesus--for obvious reasons. But if we think of ourselves as better than atheists because we call ourselves "Christians", but the atheist, by means of their conscience, does what is right and we ourselves do not; then calling ourselves "Christian" becomes entirely meaningless. And we shall--and rightly so--be condemned doubly.
"The master of that servant will come on a day he does not expect and at an hour he does not anticipate. Then He will cut him to pieces and assign him a place with the unbelievers. That servant who knows his master's will but does not get ready or act according to His will, will be beaten with many blows. But the one who unknowingly does things worthy of punishment will be beaten with few blows. From everyone who has been given much, much will be required." - Luke 12:46-48
If you think "Oh, so the atheist does something right?" is a good rebuttal, then you should take the time to go back and read your Bible more. If you think being a Christian is a defense against ungodliness, then that is not only deeply flawed, it's dangerous to your eternal health.
Grace is not an excuse to live licentiously. The Day is coming, and we must give an answer.
The Christian ought to live with holy fear. "For it is a terrible thing to fall into the hands of the living God." The Christian who thinks he shall be spared, but the unbelieving shall be condemned has no fear of God, and is no better (but certainly may be worse) than the unbeliever they wish to judge. For there is only one Judge. And we aren't Him. "For with that same judgment you judge, you too will be judged."
-CryptoLutheran