I cannot tell you how many times I have heard a Christian say, "Let your conscience be your guide." That sounds like good advice, right? One verse, Jeremiah 17:9, should be enough to show that man's conscience can not be trusted to prevent sin: "The heart is deceitful above all things, and desperately wicked; who can know it?"
After Adam sinned, he and his wife knew they were guilty, and they hid from God (Gen. 3:7-8). In my February 1, 2010 blog, I covered the Dispensation of Conscience, which was in effect from Adam's sin until the Flood, so I will not take the time to cover it here. It is obvious that man's conscience is not capable of making him good. What it is designed to do, is reveal that he is not good. Had it been able to control man's actions, there would not have been five more dispensations, which by the way, would not accomplish the task either. It is ironic that the Dispensation of Conscience lasted approximately twenty-three hundred years, and yet the words "dispensation" and "conscience" do not appear in the Old Testament. "Dispensation" appears four times in the writings of Paul, and "conscience" appears thirty times throughout the New Testament.
The Bible describes a man's conscience as being one of the following: evil (Heb. 10:22); defiled (Titus 1:15); seared (1 Tim. 4:2); weak (1 Cor. 8:7); good (Heb. 13:18); and pure (1 Tim. 3:9). Since unsaved man is at enmity with God, his conscience powerless to inhibit him (Gen. 3:15; 6:5; Rom. 8:7). Sin defiles ones conscience making it "out of tune" with the things of God, often resulting in a reprobate mind (Titus 1:15; Rom. 1:28-32). The fact that this person's conscience has become defiled, implies that it was pure at one time. A Christian whose actions disregard the conviction of the Holy Spirit, is destined to suffer in this life and in the life to come (Heb. 12:6; 1 Cor. 3:13-15; 11:30).
Some Christians have a weak conscience due to a lack of knowledge of God's Word; they are not aware of their freedom in Christ (Basically, the whole Book of Galatians, especially 2:4; 5:1; 1 Cor. 8:1-13). They are described as being tossed to and fro by every wind of doctrine (Eph. 4:14; Jam. 1:6). Paul calls them babes and lacking in the "meat" of the Word of God.(1 Cor. 3:1-2; Jn. 4:31-34). The solution for the weak Christian is to hunger for God's righteousness, the recipe for which is found in His Word (Matt. 5:6; Ps. 119:123, 172; Heb. 5:13).
I am not certain as to the difference between having a good conscience and having a pure conscience. It seems to me that when a mature Christian lives his or her life according to the Word of God, illuminated by the Holy Spirit, both are true. The Word, without the guidance of the Spirit, is often used of man to abuse others. False prophets and false teachers of the Word are guaranteed to lead weak Christians astray. Feed your conscience on His Word, and you will recognize them for who they are: agents of Satan. "Thy Word have I hid in mine heart, that I might not sin against Thee" (Ps. 119:11). Feed your conscience on the meat of the Word!