“But if you call yourself a Jew and rely on the law and boast in God and know his will and approve what is excellent, because you are instructed from the law; and if you are sure that you yourself are a guide to the blind, a light to those who are in darkness, an instructor of the foolish, a teacher of children, having in the law the embodiment of knowledge and truth— you then who teach others, do you not teach yourself? While you preach against stealing, do you steal? You who say that one must not commit adultery, do you commit adultery? You who abhor idols, do you rob temples? You who boast in the law dishonor God by breaking the law. For, as it is written, “The name of God is blasphemed among the Gentiles because of you.”” -Romans 2:17–24
In verses 19-2, having addressed their privileges as Jews, Paul addresses their perspective about their privileges. The imaginary Jewish interlocutors Paul is addressing are “sure” that their place in the world is to be a guide to the blind, a light to those in darkness, an instructor of the foolish, and a teacher of children. In fact, there is a sense in which they are not wrong.
“I am the Lord; I have called you in righteousness; I will take you by the hand and keep you; I will give you as a covenant for the people, a light for the nations, to open the eyes that are blind, to bring out the prisoners from the dungeon, from the prison those who sit in darkness.” -Isaiah 42:6–7
“And nations shall come to your light, and kings to the brightness of your rising.” -Isaiah 60:3
God had given the Jewish nation the revelation that was to be distributed to all nations, the revelation of the law, which is the embodiment of knowledge and truth. Their privilege wasn’t to be an end in itself; their “chosen” status was like that of the apostles, for the purpose of teaching all mankind about God. (See John 6:1-14 for a beautiful picture of the Jews purpose to this end.)
Their condemnation, Paul is about to point out, is that they who taught others failed to teach themselves.