If you know some action is wrong, what difference does it make if you don’t know why it is wrong or not? I mean why is black not white? If you know some action is wrong and thus a sin and evil, than we know immediately we ought not do the action whether or not we know the action could be potentially mortally sinful or only venially so. The first principle of the natural law is ‘do and pursue good and avoid evil’ and God doesn’t command us to sin but rather to not sin and avoid evil.
For a catholic whose faith is founded on divine revelation, it is enough that we know that some action is gravely or mortally sinful in its object on the authority of God revealing who cannot error regardless of the why of it. It is simply our duty to obey and do God’s will wherein the perfection of the christian life consists. For the perfection of the christian life in this life here on earth consists essentially in charity or love of God and our neighbor which is in the will for love is an act of the will. The theological virtue of faith pertains to the knowledge God has revealed about himself and about what we must do and avoid such as in the ten commandments and to unite our wills to His in charity. But, faith is not a ‘perfect’ knowledge of God in this life and it will give way to sight in heaven where we will see God face to face. So, here on earth we walk by faith and not by sight. We can grow in our knowledge of God by studying the faith and in prayerful reading of Holy Scripture.
The Church teaches that "mortal sin is sin whose object is grave matter and which is also committed with full knowledge and deliberate consent (CCC#1857)…Grave matter is specified by the Ten Commandments…It presupposes knowledge of the sinful character of the act ( CCC#1857-1859). The CCC tells us, though not exhaustively, which human actions constitute grave matter in their object such as murder, adultery, fornication, contraception, etc. Accordingly, if a person knows, for example, adultery is a sin as well as knowing adultery constitutes grave matter in its object or in itself, intrinsically evil, as the CCC teaches, than such knowledge in my understanding satisfies the ‘full knowledge’ aspect of mortal sin. The only other aspect to consider is deliberate or complete (full) consent.
Perfect and certain knowledge of ‘why’ adultery is objectively a mortal sin is simply founded on the authority of God telling us by revelation. Of course, with the natural light of human reason we can provide arguments why adultery is wrong as it is against the natural law. God is both the author of the natural law and divinely revealed law such as the Ten Commandments God revealed to the Israelites through Moses. But, the Divine Reason or divine revelation is more certain than human reason which can error concerning precisely what the natural law specifies which is why God revealed his law and commandments to us so that all men might know with certainty God’s will for us concerning moral acts and our destiny.
