God provided a conscience even in unregenerate men to point them toward His principles and ways.  All of us, before we are saved would be totally depraved if it were not for this provision of God by His grace. However, the longer people live in sin and carnality, the less the true inward conscience has any authority in their lives. Most of the time, particularly in today's culture, we have developed "false" consciences. These are "programs" in our minds, not our spirits, as to what is right and wrong, and these things (evolution, secularism, humanism, situational ethics, and traditions and doctrines of religious thinking) control what we think is moral and ethical. The world's thinking overrides and drowns out the inner voice of the true conscience that God put within us.

Hebrews 10:22 "Let us draw near with a true heart in full assurance of faith, having our hearts sprinkled from an evil conscience, and our bodies washed with pure water.
23 Let us hold fast the profession of our faith without wavering; (for he is faithful that promised;)
24 And let us consider one another to provoke unto love and to good works."

Because of the world's ungodly influences, it is very important for a Christian to allow His conscience to be "healed," or renewed, to reflect God's standards which we find in the Bible. That is why Christians cannot be guided by their consciences in most cases. It is much safer to be guided by the Word of God and by witnesses and guidance from the Holy Spirit.

Today we are shocked by the evil acts of men who perform ghastly deeds and show no remorse whatsoever. We ask, "How can they do such things?" When the conscience is seared -- burned out by constantly going against its nudgings -- then men become like animals in their behavior. The Bible speaks of this in the following verses.

1 Timothy 4:1-2: "Now the Spirit speaketh expressly, that in the latter times some shall depart from the faith, giving heed to seducing spirits, and doctrines of devils; 2 Speaking lies in hypocrisy; having their conscience seared with a hot iron."

This also happened in ancient times, according to Paul's letter to the church at Rome (Romans 1). After a certain period of time, God will give someone with a seared or evil conscience "over to a reprobate mind."  That means the mind cannot be renewed, because the person has for so long chosen self-ways (which really are Satan's ways) over God's that it is not possible for them to change. This should be a warning for all who persist in sin.

Romans 1:28-32: "And even as they did not like to retain God in their knowledge, God gave them over to a reprobate mind, to do those things which are not convenient;
29 Being filled with all unrighteousness, fornication, wickedness, covetousness, maliciousness; full of envy, murder, debate, deceit, malignity; whisperers,
30 Backbiters, haters of God, despiteful, proud, boasters, inventors of evil things, disobedient to parents,
31 Without understanding, covenantbreakers, without natural affection, implacable, unmerciful:
32 Who knowing the judgment of God, that they which commit such things are worthy of death, not only do the same, but have pleasure in them that do them."

When we give our lives to the Lord, He begins a work in our hearts and minds and brings healing and enlightenment to the conscience.When a Christian's conscience is renewed, it will not condemn them. When a Christian's mind is renewed, it will not think carnal, self-centered thoughts. When a Christian's emotions are renewed, they will no longer be negative. When a Christian's will is renewed and submitted to God, he will no longer choose against God's ways.