...in that they show the work of the Law written in their hearts, their conscience bearing witness and their thoughts alternately accusing or else defending them, on the day when, according to my gospel, God will judge the secrets of men through Christ Jesus. Romans 2:15-16 (NASB)
Have you heard someone say, "Let your conscience be your guide?" I have, and it is almost as bad of advice as the infamous, "Follow your heart" type of counsel I hear so many promoting today.
While it is true that our conscience is God-given, and can assist us in determining right from wrong, good from bad it may not be a good or biblical plan to let your conscience be your guide. Your conscience does not operate in a vacuum and it is affected by the culture and the world around you. It is influenced by the news, magazines, and the people that surround you. It is also influenced by your church, the preaching you sit under, and the teachers in your Sunday school class.
For example, the first church I attended after being regenerated was a little on the legalistic side, although I didn't know it at the time. They were very "do this, and don't do that" oriented. Whether we realized it or not, the attitude was that if you did something we didn't do (like listen to contemporary Christian music and we listened only to hymns) then we were better Christians than you. On the other side, if you didn't do something that we did (like door to door evangelism) than we were better Christians than you. There is nothing in Scripture that speaks to either thing, but that didn't stop us from judging.
The conscience of the legalist is based on Law and their own interpretation of that law. They tend to carry a tremendous amount of guilt because they are unable to live up to their own legalistic standards. They become slaves to traditions and the opinions of men (Gal. 5:1-6) and lose sight of the grace of God. Two heavy hitting areas in our current age would be that of the kind of music you listen to, and if you have a tattoo or wear gauges in your ears.
The conscience of the young in Christ, or the new believer is usually described of as being "weak." This is because they are not knowledgeable enough in the faith to understand right from wrong in some cases. Their sin is not a done out of rebellion, but ignorance of what God's Word says on a subject.
This was the case in 1 Corinthians 8 as Paul instructed them on eating food sacrificed to idols. They were ignorant of what Paul's teaching was on this subject and on the results of what disastrous results could take place in the lives of the weaker brethren. A current example would be drinking alcohol or dating a non-Christian. Until a conscience is informed as to what the Scriptures say about these topics there can be no biblical understanding.
The most devastating about a conscience is when it is what the Bible calls "seared."
...by means of the hypocrisy of liars seared in their own conscience as with a branding iron, 1 Timothy 4:2 (NASB)
A seared conscience is one that has been pricked by the truth of the Word of God but that Word has not been obeyed. The Spirit of God has been active in the person's heart but denied so many times that the heart has grown callous and hard to the "voice" of the Lord. This is how a person who is a Christian can commit a certain sin for a long period of time and be seemingly unaffected by guilt. The person can be very obedient in many other areas of their life but not in this one area. For example, many Christians thinking nothing of saying "Oh my God!" or using the current "OMG!" in every day conversation. It is quite possible that the conscience is seared in this area.
This is why it is critical to study the Word and learn what Scripture has to say on the issues of life. It is the only place we can learn what God's perspective and expectation is on a topic. There are places where God very clearly tells us His expectations and we would do well to learn them and obey them. There are warnings against living under the law and legalism in light of the freedom we have in Christ, and just as many warnings against living as though our freedom was not a costly thing.
It is good to know that God always tells us how to do what He asks us to do. We do not need to live in confusion or under the weight of self-imposed regulations or law that others would press upon us. We can, by God's grace and the truth of the Bible-
...deny ungodliness and worldly desires and to live sensibly, righteously and godly in the present age, looking for the blessed hope and the appearing of the glory of our great God and Savior, Christ Jesus, who gave Himself for us to redeem us from every lawless deed, and to purify for Himself a people for His own possession, zealous for good deeds. Titus 2:12-14 (NASB)