In explaining that everyone has the moral law within them, Paul says in Rom. 2:14-15 that the Gentiles are a law for themselves even though they don’t have the law (Scripture) “since they show that the requirements of the law are written on their hearts, their consciences also bearing witness, and their thoughts now accusing, now even defending them.”
But how is the conscience both an accuser and a defender? If a man leaves his wife for another woman, his conscience will accuse him of his wrong. How do we know he feels accused? The noble reasons he uses to defend himself give him away. He may say, “I wasn’t being honest to myself…I never loved her,” or “I found the courage to follow my heart,” or “She’s deserves better than me.”
A conscience can be hardened, seared, ignored, or suppressed, but it usually doesn’t remain silent. And even if it becomes severely damaged, God gave us an external standard, the Bible, to recalibrate it.