In this article we will talk about conscience, its origin, constitution and purpose, as well as its action and its effects on individual souls. We will also discuss the manner in which a soul may purify his conscience, that is be able to hear and obey it with greater clarity, and warn against those actions which cause us to become deaf to our conscience.
But above all they must KNOW which of their actions have been mortal sin. Enter the Natural Law and conscience:
There are many fundamental basics of morality that are universally recognized in every culture and in every society, which is a testament to the existence not only of a conscience, but of a universally calibrated conscience (we quote Aristotle to show that the conclusions of the natural law are indeed capable of fruition in men's minds by natural reason alone, without the aid of Christian theology). Even the native of a faraway land who has never heard the Gospel has a conscience, which tells him right from wrong.
Pope Pius IX, Vatican Council, Canons on Revelation, #1, ex cathedra: "If anyone says that the one, true God, our creator and lord, cannot be known with certainty from the things that have been made, by the natural light of human reason: let him be anathema"
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St. John 3:5, 11:26: "Unless a man be born again of water and the Holy Ghost, he cannot enter into the kingdom of God, [...] And every one that liveth, and believeth in me, shall not die for ever."