Albert Barnes on Romans 2:19

And art confident. This expression denotes the full assurance of the Jew that he was superior in knowledge to all other people. It is a remarkable fact, that the Jews put the fullest confidence in their religion. Though proud, wicked, and hypocritical, yet they were not speculative infidels. It was one of their characteristics, evinced through all their history, that they had the fullest assurance that God was the Author of their institutions, and that their religion was his appointment.

A guide of the blind. A guide of the blind is a figurative expression to denote an instructer of the ignorant. The blind here properly refers to the Gentiles, who were thus regarded by the Jews. The meaning is, that they esteemed themselves qualified to instruct the heathen world, Mt 15:14, 23:16.

A light. Another figurative expression to denote a teacher. Comp. Isa 49:6, Jn 1:4,5,8,9.

In darkness. A common expression to denote the ignorance of the Gentile world. Mt 4:16.