12 For as many as have sinned without law shall also perish without law: and as many as have sinned in the law shall be judged by the law; 13 (For not the hearers of the law are just before God, but the doers of the law shall be justified. 14 For when the Gentiles, which have not the law, do by nature the things contained in the law, these, having not the law, are a law unto themselves: 15 Which shew the work of the law written in their hearts, their conscience also bearing witness, and their thoughts the mean while accusing or else excusing one another;) 16 In the day when God shall judge the secrets of men by Jesus Christ according to my gospel.
Paul used the first part of this chapter to establish that the Jews, who had the law, were guilty before God because they had violated the law. Now in this section Paul sets out to show how the gentiles, without God’s law, would still be judged as guilty by God. This is what he means by "For as many as have sinned without law shall also perish without law: and as many as have sinned in the law shall be judged by the law". The question is often asked, “How can God hold people accountable that have never heard His law or read the scriptures?”, Paul answers it here.
Even though men are born in a sinful state of depravity God has not left them without an imprint of His law on their hearts. We see this in society all around us. From the individual perspective men are born knowing that lying, stealing, and many other things are wrong. When a child lies to their parents that child knows in their heart it is wrong to lie yet they do it anyway. We also see this from the collective perspective of nations and states. Even the most godless nations in history have had laws against murder, stealing, rape, and other such acts. This is because God has left an imprint on the hearts of depraved men of what is right and what is wrong. We call it a conscience.
Paul states here that the conscience that men are born with will either accuse them or excuse them. The simple meaning of this is that men without the written Old Testament law have violated their own moral law that God has written on their hearts. There is not a person in the world that can say that they have not done things that their own conscience has told them was wrong, this is what Paul means by the conscience “accusing” them. Those that are without the law God can judge based on the simple fact that they have down things they knew were wrong and “excused” themselves for doing them.
When men without the law of God stand before God on judgement day God can show them the acts they did that violated their own conscience as a judgement against them and without the grace found in Christ they will be just as guilty as those who had the law and violated the law. Every act, even the ones people thought were done in secret, will be judged. Paul uses this chapter to show that the whole world stands guilty before God.