Human understanding is “darkened” (Eph. 4:18), but the conscience is the area where God has given light to every man. The word “con-science” means “with knowledge.” The conscience is the headline warning of sin; the Scriptures give the fine print. None of us can say that we don’t know it’s wrong to lie, steal, murder, or commit adultery; that knowledge is written in large print on our heart. However, in the Scriptures we see the true nature of sin: that God requires truth even in the inward parts (see Psalm 51:6). The fine print reveals that lust is adultery of the heart, hatred is murder of the heart, fibs are bearing false witness, etc.
“Conscience is the internal perception of God’s moral Law.” Oswald Chambers
Romans 2:15 who show the work of the law written in their hearts, their conscience also bearing witness, and between themselves their thoughts accusing or else excusing them.
Ray Comfort's Daily Evidence 