What does Romans 2:18 mean?

Paul is building a devastating if-then statement in this section of verses. He is addressing those who would call themselves Jews, who rely on the law, and who boast in God. The prior verse referred to those who identified as Jewish, depending on the law and confident in their relationship to God.

Now he further defines the people he is talking to. These people do not simply rely on the law of Moses. Through it, they know God's will, and they use His law as a standard by which to decide if anything is excellent. They can do this because they have been so well taught from Moses' law.

After building up this group for all their knowledge and understanding, Paul will challenge their actual behavior in the following verses. The larger point he is making is about their misplaced confidence. These people have confidence that God will not judge their sin, because of their identification with the law. This is assurance aimed in the wrong place. Paul uses this idea to show how the Jewish people are as guilty as the Gentiles, and must be saved by grace and not the law.