A "Good" Conscience
Some engines are built to run at a certain speed; and to maintain a constant rate of speed a mechanical gadget, called a "governor," is used. When properly installed and adjusted the governor will retard the engine if it tends to "run away", or it will speed up the engine if it labors.
If the governor is improperly set it will, of course, cause the machine to run incorrectly and perhaps ruin it. But in such a case we could not blame the governor- which performed exactly as it was set to perform. We must determine who improperly adjusted the governor, and lay blame there. The governor is not the standard of right and wrong; it is simply a sort of mechanical police force, that acts according to predetermined rules.
Now if you can understand the difference in this mechanical gadget and the "authority" by which it is set; you can understand the difference in man's conscience, and the standard of truth by which it must be "set" if it is to "prick" us at the right time. Thousands of people rely on "follow you conscience" -- and never give a thought to the adjustment of their conscience, nor to the rule book by which that adjustment may be checked.
When Paul "verily thought" with himself that he ought to do many things contrary to the name of Jesus of Nazareth (Acts.26:9-) he acted -- "in all good conscience before God" (23:1, 24:16). It didn't hurt his conscience to persecute Christians, because his conscience was so adjusted. But his honesty did not justify his actions. His Jewish background: his upbringing, and education, had shaped his thinking -- even as our background shapes our thinking. He was "honest with himself" (had a good conscience) but was honestly mistaken. If having a clean conscience is all that is necessary to go to heaven there was no need for Christ to die, no need for the Bible, no need for anything commanded by God. Think that over, and try to "be honest with yourself". LOTS OF FOLK ARE HYPOCRITICAL WHEN TALKING A GOOD CONSCIENCE.
When Paul learned the truth about Christ he had to act accordingly -- to keep a good conscience. Conscience is not truth, but a governor which urges us to act according to our conception of truth. As children, our parents taught us as best they could, and our childhood environment molded our conceptions of right and wrong -- even affecting deep-seated moral tendencies. But God's revealed truth is our standard (Jn.17:17; 2 Tim.3:16-). As adults accountable to God, we must now think for ourselves and "adjust" our conscience by an honest appeal to God's standard of truth.
The conscience itself is not the standard for anything; its important task is to goad us into action that accords with our very best and most honest knowledge. It is a governor, a piece of moral mechanism that comes as standard equipment on every intelligent human being. Treat it rightly, and it will serve you well; but sear it (1 Tim.4:1-) or use it as excuse to ignore God's message (the Bible) and you'll "believe a lie" and be damned (Read 2 Thes.2:10-14).