To explain this verse, we will need to go over verses 12-14 that lead up to verse 15. I will quote each verse with brief comments.

Verse 12 says, “For as many as have sinned WITHOUT LAW will also perish without law, and as many as have sinned IN THE LAW will be judged by the law.” We see two classes of sinners here.

1)  The GENTILES who were never “under the law” but were still guilty of sinning and will perish because of their sinful deeds. As we shall see when we get to verse 15, they did have something that was convicting them of sin: they had a CONSCIENCE, yet they never acted on their conscience when it told them they were sinning.

2) The JEWS did have the Law (i.e. the Ten Commandments) given to them and were told to obey it. They had told Moses BEFORE He went up Mount Sinai to receive the Law, “All that the LORD has spoken WE WILL DO” (Exodus 19:8). Little did they realize that they would be unable to obey the Law perfectly because of their sinful hearts and unless they were convicted by the law of their sin (see Romans 3:19-20), they would be “judged by the law that they had broken.”

Verse 13: “For not the hearers of the law are just in the sight of God, but the doers of the law will be justified;” The Jews who had the Law weren’t exempt from being judged based on RECEIVING THE LAW; they were responsible to OBEY THE LAW,” which simply means they had to be “doers of the law.”  If one was able to obey the Law perfectly (from the time they were born to the time of their death), he/she would be JUSTIFIED (declared righteous by God). But as we saw earlier, the Law was meant to “convict them of sin” and show them that they could NOT be “justified by the Law” (see again Romans 3:20 along with Acts 13:39; Galatians 2:16, 21; 3:11).

Verses 14-15: “For when Gentiles, who do not have the law, by nature do the things in the law, these, although not having the law, are a LAW TO THEMSELVES, WHO SHOW THE WORK OF THE LAW WRITTEN IN THEIR HEARTS, their CONSCIENCE also bearing witness, and between themselves their THOUGHTS accusing or else excusing them.” The basic meaning of this is that the Gentiles who had never heard of the Law of Moses, still had a CONSCIENCE that “accuses or excuses” their actions. They can instinctively know (by the CONSCIENCE that was given to man after Adam ate of the “tree of the knowledge of good and evil”…Genesis 2:15-17 and 3:1-6), that they have done something wrong. If they MURDER someone or STEAL from someone or LIE to someone, their conscience (that “still small voice in our minds”) will “accuse them.” STEALING, LYING, and MURDER were “three of God’s commandments” that were written on tables of stone” (Exodus 20:13, 15, 16), but through the CONSCIENCE we learn that “the Law is also written on men’s hearts.” On Judgment Day the unbeliever will not be able to say to God: “But I never heard of the Ten Commandments or the gospel of Jesus Christ.” They will be “without excuse” for not acting on the “LIGHT of conscience” or, I might add, the “LIGHT of creation” (see Romans 1:18-20).

It should be said that the Law written by the finger of God is the “greater LIGHT,” for it is part of His Word which is “inerrant” (incapable of error). The CONSCIENCE can become DEFILED (Titus 1:15) and even worse it can become SEARED (1 Timothy 4:2), which means it may not be able to convict one of “what is right and what is wrong.” Isaiah 5:20 speaks of this: “Woe to those who call evil good, and good evil.” Yet men will be held responsible for not listening to their conscience when it was functioning and “accusing them of sin” and of allowing their conscience to quit working.  (DO)  (725.5)