They show that the work of the law is written on their hearts, their conscience bearing witness alongside, their thoughts accusing them or even speaking in their defense
In context
13It is not the hearers of the law who are righteous before God; it is the doers of the law who will be justified. 14When Gentiles, who have no law, do by instinct what the law requires, they are a law to themselves, though the law is not theirs. 15They show that the work of the law is written on their hearts, their conscience bearing witness alongside, their thoughts accusing them or even speaking in their defense 16on the day when God judges what people keep hidden, through Christ Jesus, according to my gospel. 17Now then: you call yourself a Jew, you lean on the law, you boast in God;