How the Conscience Works
Once upon a time in a garden, Adam and Eve chose to disobey God and eat from the tree of the knowledge of good and evil. From that knowledge was derived our conscience that has since been handed down to all mankind.
Like smoke damage inside a house, the alarm alerts us to the defilement that is occurring as the alarm is sounding. Nobody but God can see this smoke damage on the soul, but the home owner senses it and feels unclean in his guilt (Is.1:18).
There are Laws of God we are born with and Laws that God has added, primarily as part of the OT ceremonial system:
1)Laws We are Born With: (Rom.2:1-3).
These ceremonial laws were abolished when Jesus ushered in the New Covenant. But the conscience of many Jews, steeped in these ceremonial laws, did not embrace the New Covenant as readily as did Peter’s conscience (Acts 11:6-9). Gentile Christians were therefore admonished to follow certain of these laws when they were in the company of Jews just in case some Jews would be tempted to eat something in violation of their weak conscience (Acts 15;20).
As for an unbeliever, his conscience can lose its sensitivity when repeatedly violated (1Tim.4:2)---its sensors becoming clogged with the the soot of sin and no longer sounding an alarm when tripped by evil (Titus 1:15).