ANSWER: It is the faculty which discovers the moral quality of actions, and approves the right and condemns the wrong. On abstract questions its voice is always the same in all countries and in all generations, such as, Is it wrong to hate a benefactor, or punish innocence? These are questions relating to immutable morality. But most of the questions we meet with are not abstract and simple, but concrete and mixed, requiring the exercise of our fallible intellects, so that one may say that an act is right and another say it is wrong. That is the field of mutable morality. Hence the need of a well-trained intellect illuminated by a good knowledge of God's Word, especially of the New Testament. Such a conscience is our guide, not infallible in the field of practical life, except in the case of the Pope, if we believe that he is the divinely appointed organ of the Holy Spirit who cannot err. The best guide is a tender conscience very sensitive to moral distinctions, like a pair of scales so delicately poised as to weigh a hair. The worst is a seared conscience (1 Tim 4:2), which by being habitually disregarded has now lost its sensitiveness, as flesh cauterized till it has ceased to feel. Such a guide leads to the pit of woe. The only remedy is a supernatural change wrought by the Holy Spirit regenerating and sanctifying. A good conscience is a tender, moral sense, which approves our conduct, and a bad conscience is one that condemns.
— From Steele's Answers pp. 15,16.