Before the fall, strictly speaking, there was no conscience in humans. There was no gap between what they were and what they knew they had to be. Being and self-consciousness were in harmony. But the fall produced separation. By the grace of God, humans still retain the consciousness that they ought to be different, that in all respects they must conform to God’s law. But reality witnesses otherwise; they are not who they ought to be. And this witness is the conscience. The conscience … is proof that communion with God has been broken, that there is a gap between God and us, between his law and our state. … The human conscience is the subjective proof of humanity’s fall, a witness to human guilt before the face of God.
Reformed Dogmatics Volume 3
I think that Bavinck and some responders are using the narrowest definition of ‘conscience’: Adam and Eve had no “guilty conscience” pre-Fall. But the actual function of conscience/suneideesis is a creational given, intrinsic to the image of God. It is our entire evaluative capacity, not only about ourselves but about everything we encounter. Is that good or bad? Is that true or false? Is that valuable or worthless? We humans don’t only know things, but we weigh the things we know. We are meant to love what is true, good and beautiful, and to hate what is false, wrong and shameful. When the serpent told lies, their consciences should have said, ‘That is a lie. That’s wrong and deadly.’ This is one of many areas where we need to understand the flexibility of biblical language in capturing both narrower and broader meanings. The Old Testament equivalents to this comprehensive functioning of conscience are such phrases as ‘in my eyes’ (or ‘your eyes’) and ‘the ear tests words as the palate tests food.’ The Bible actually uses the term conscience and its equivalents far more often in the general sense than in the narrow sense.
Via: Tony Reinke