From Herman Bavinck in his Reformed Dogmatics, 3:173:
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.
Tony Reinke 
Interesting way of looking at it! So, someone who doesn’t have a sinful nature needs no conscience, because they aren’t separated from God. It’s only those with the capacity to stray that feel the twinge of a conscience telling them when they’ve fallen short. And yet it is this very thing that is proof that they need a savior. Never thought of that before…
Interesting way of looking at it. Seems logical and makes senses but Bavinck’s View does have a hard time with Romans 2:12-16, especially 2:15. Seems Adam did not lack a conscience, but he had a perfect conscience.
Good thought, brother. I think Bavinck overstates it when he says pre-fall Adam had no conscience. However, I think he makes an important point in this quote. Did Adam have the faculty to distinguish between good / evil from day one? I’m still thinking about this, but in light of Gen 3:5, 22 I am unsure that Adam had an awakened conscience. My current understanding of the fall is that it was a failure to obey God’s command rather than failing to make a proper decision between good / evil. I’m not sure Adam had this capacity, this activity of conscience, because there was no knowledge of the distinction between good/evil and without that knowledge there is no conscience (Bavinck). At least this is one question that comes to my mind. But alas, Pro 30:2.
“I am unsure that Adam had an awakened conscience. My current understanding of the fall is that it was a failure to obey God’s command rather than failing to make a proper decision between good / evil.”
Maybe what David Murray taught me can help us understand this as well. “Conscience is not just a negative (telling what we did wrong), but a positive, telling us what is right to do and approving what is done right. Look at Romans 2:15. It is not only accusing but defending. That’s what makes Adam & Eve’s sin even worse. They not only had God’s command, they had clear and clean consciences as well.