[A]ll that can be known of God lies plain before their eyes; indeed God himself has disclosed it to them. Ever since the world began his invisible attributes, that is to say his everlasting power and deity, have been visible to the eye of reason, in the things he has made. Their conduct, therefore, is indefensible; knowing God, they have refused to honor him as God, or to render him thanks.
When gentiles who do not possess the law carry out its precepts by the light of nature, then, although they have no law, they are their own law; they show that what the law requires is inscribed on their hearts, and to this their conscience gives supporting witness, since their own thoughts argue the case, sometimes against them, sometimes even for them.
We can know God by what he has made, to the point where we should know enough to honor him and render him thanks. In other words by observing creation we can draw the conclusion that God exists and is deserving of honor and praise. What the law requires is inscribed on our hearts, and our conscience bears supporting witness.
I learned in a recorded lecture by Fr. Brian Mullady, O.P., that conscience is basically a syllogism. (This seems to be consistent with what St. Paul says, that “their own thoughts argue the case, sometimes against them, sometimes even for them.”) As I understand it, the major premise of the syllogism is whether or not a certain type of act is moral, as a general principle; the minor premise is whether or not the act presently contemplated constitutes an act of the type indicated in the major premise; the conclusion is whether or not the act should be done. Thus,
A. Artificial birth control is immoral
B. Use of a condom is artificial birth control
C. Therefore, I should not use a condom
Where many people go wrong in thinking of conscience, is in believing that they themselves are the source of A. Whereas actually, their conscience is simply the process by which they reason from A to the conclusion whether or not they may perform a certain act. The source of A is God’s revelation, either through scripture and the Church, or through his creation.
In other words a Catholic, knowing that the Church has taught that artificial birth control is wrong, may say to himself, “But I am told to act according to my conscience, even if it differs from what the Church teaches. And my conscience tells me that artificial birth control is not wrong.” Thus he thinks his conscience is the source of A.
But it’s not the role of the individual’s conscience to be the source of A, representing general principles of morality. The role of the individual’s conscience is only to apply A to the specific situations he encounters in his life. Through a mistake in reasoning he may innocently arrive at the conclusion that he may perform a specific act at a specific time, which is actually objectively sinful. But he may not innocently decide to make himself the source of A, usurping God’s revelation of the moral law given through the Church.