A person's conscience is his thoughtful application of accepted moral principles to decide the moral goodness or badness of an imminent action. It's the practical intellect engaged in the act of judging what ought to be done or not done in an immediate moral choice.
The conscience applies objective moral principles it gains by faith or revelation and from reason or naturally known moral truths. Examples of such principles are "do good and avoid evil" and the Ten Commandments. Catholics are obliged to adopt Catholic moral principles when informing their conscience as demanded by canon 750 in the Code of Canon Law. Failure to do so is a rejection of Christ as He said in Luke 10:16, "He that hears you hears Me."
Failing to properly inform one's conscience with morally sound principles results in following a blind guide. Informing it with immoral principles results in following an evil guide. A deformed conscience erroneously accepts as true immoral principles such as "do evil that good may come of it" or rationalizations such as "look out for number one." A Catholic's conscience applies Catholic moral principles in the context of personal circumstances without dissenting from those principles.
Violating one's conscience occurs in failing to apply the moral principles it has previously adopted. This results in a guilty feeling sometimes called a "twinge of conscience." Repeated violations of one's conscience result in a deadened conscience or "hardened heart," whereby the sense of guilt is numbed. The mere absence of guilty feelings, however, doesn't mean a person isn't guilty from God's viewpoint.
In 1946, Pope Pius XII noted the "loss of the sense of sin" was the greatest sin of modern time. In his 1984 apostolic exhortation, Reconciliatio et Paenitenia, Pope John Paul II referenced Pius XII's statement, adding that the "loss of the sense of sin" deceives man into accepting "an illusion of sinlessness."
Learn how Canadian shepherds deformed the consciences of their flock, starting with birth control in this episode of Church Militant's Premium show, FBI—Canadian Collapse.