Paul responds to the fact that Gentiles did not receive the law through Moses by saying that they have the law of God written on their hearts. Earlier he had reminded his readers that Gentiles receive day by day what we call natural revelation, that they see evidence of the existence of God’s power in creation. Now Paul adds another detail to what Gentiles possess from God and that is that each of them has God’s law written on their hearts. They not only know that he is powerful, they know that he is their lawgiver.
In order to help understand this, we can think of it in this way, even if the Gentiles did not have the law in written form. We know that the law of God is summarised as love for God and love for our neighbour. What happens when we take love for God out of the process? We lose the main reason for loving our neighbour and have to invent other reasons why we should do so.
The Gentile world does not love God, says Paul in Romans 1, even although he speaks to them in creation. Therefore, they don’t take him into account in their daily lives. But they have to live with one another and sometimes their actions with one another resemble the law of God whereas at other times their actions with one another don’t resemble the law of God. The actions that resemble the law of God are evidence that his laws are still written on their hearts and the actions that don’t resemble the law of God are evidence that they rebel against him.
Moreover, Paul says that each of them has a conscience that informs them that what they are doing is right or wrong. The conscience functions like a spokesperson for God. It functions differently according to circumstances. Normally the conscience has an opinion on all that we do and it is determined to let us know what that opinion is.
Paul says that conscience will speak throughout life and that it will speak on the Day of Judgement. Today, the reasons why people do things are secret from everyone else. On that future day, they will be public and one of the witnesses will be the conscience. I suppose we can imagine a situation in a courtroom where a witness has been battered and bruised by the accused throughout life. That may be a picture of how many a conscience will appear on the Day of Judgement, but the point is that it will appear. And it will tell its owner that he or she deserves the verdict pronounced by the Judge.