‘(For when Gentiles who do not have the law do by nature the things of the law, these, not having the law, are the law to themselves, in that they show the work of the law written in their hearts, their conscience bearing witness with it, and their thoughts one with another accusing or else excusing them),'
Paul's flow of argument suddenly comes to a halt as he recognises that someone will therefore object, ‘but if the Gentiles are not under the Law (Romans 2:12), how can they be judged by the Law (Romans 2:13)?' So he now explains how that is so.
These two verses are to be seen as in parenthesis. They interrupt the flow of the narrative in order to explain how the Gentiles could be judged by law (Romans 2:13) when they were without law Romans 2:12. Why, says Paul, they do have law, for you will notice that the Gentiles who do not have the Law, do by nature the things of the Law, thus demonstrating that they have the equivalent in themselves, that they are following their own inner law, a law to which their conscience bears witness. Such people are a law to themselves. For by their moral actions and behaviour they show the work of the Law written in their hearts, and their conscience bears witness with it. This is demonstrated by the fact that they are constantly arguing the moral case for things, sometimes approving of them and sometimes disapproving. Sometimes accusing and sometimes excusing. In other words they demonstrate a moral dimension in their lives in which both positive and negative positions can be arrived at, showing that some kind of law is at work.
The idea of the law written in the heart is found in Jeremiah 31:33, but there the idea is of the living laws in men's hearts replacing the written Law. It is, however, seen as the same Law. Here too we have a law written by God in their hearts, a moral dimension within Gentiles which guides their ways. And it is because they have this moral dimension ‘written within them' that they can be judged by it and found guilty of breaking it.
Some do not see these verses as a parenthesis, arguing that the argument continues, but the end result is the same. Others consider that it depicts the Gentile who has become a Christian and thus has God's laws written in his heart in accordance with the words of Jeremiah. They have lived according to conscience. But the fact that these Gentiles do it ‘by nature' is against this suggestion.