The three conditions for a mortal sin have not changed and can never change, because they are based on the natural law of love.
Morality can only be understood in the context of our love affair with God. Outside of that context morality becomes a cold, lifeless, harsh set of rules imposed by an arbitrary, vindictive god.
The “Good News” of the Gospel is in direct contradiction to such a view. The good news of the Gospel is that God loves us and wants us to be happy.
The Lord himself used marriage as an analogy for his relationship with us. So I think it can be helpful to use that analogy when discussing things like sin in the context of our love affair with him.
For example, if I snap unjustly at my wife after a hard day at work, I commit an offense against her. It is a small offense and may be easily overlooked by my wife because she loves me and knows that I really do love her. She knows my offense came as a result of weakness and imperfection, not out of genuine malice or disregard for her. She may not even react or give me any indication that she is offended, having forgiven me even without my asking. Indeed, it may not even register in her consciousness that I snapped at her.
Nevertheless, my offense, particularly if it is often repeated, has caused some harm to our relationship, however minor. If I continue in this behavior without remorse and a sincere attempt to correct my behavior, it does reflect a certain lack of love on my part, and it will make it easier and easier for me to offend her in greater and greater matters.
A sincere expression of love on my part for my wife can overcome the effects of this type of offense. The next time I make love to her, it is a genuine expression of my love for her, even if it is an imperfect love.
Offenses such as this are what we mean when we refer to “venial sin.”
If, on the other hand, I were to commit adultery, this would fundamentally rupture our relationship, whether she knew about it or not. It would make it impossible for me to express a true act of love until I acknowledged my offense and sincerely sought forgiveness. The offense can be forgiven, but forgiveness requires an explicit expression of both guilt and sorrow. Indeed, if my wife were to “forgive” an act of adultery without such an expression of guilt and sorrow, she would be denying her own dignity and value as a child of God and as a wife. In a sense, she would even become complicit in my continuing to live in a state of sin, because it would be a failure on her part to “admonish the sinner.” And she has a particular obligation, based on our marriage vows, to work to bring me to heaven.
This type of sin is a mortal sin. It ruptures the bond of love between the lovers. If I were to “make love” with her before the bond of love was restored, it would not be a genuine act of love at all. It would be a lie. It is true that the bond of love can be restored, but only with a sincere expression of guilt and sorrow.
When we view venial and mortal sin in the context of love we can see much more clearly that the three conditions of serious matter, full knowledge, and full consent must simultaneously be present for a mortal sin to occur and why this is so.
What has, perhaps, changed over the course of 2000 years of Christian moral theology is not so much the nature of mortal vs. venial sin, but rather, the way in which this reality, which is based on the natural law of love, has been expressed and taught.
As the study of medicine has informed us more fully on the subtleties and pathologies of human thought and emotion, moral theologians have been given better tools with which to understand and discuss these matters. It is only reasonable that over time the Church’s ability to refine its teachings with greater and greater fidelity to the Truth will be enhanced. This is, in fact, the promise of Jesus when he told us that he would remain with the Church until the end of time.
I apologize for such a long post, but I think it’s crucial to discuss these matters in the context of love, as we know and experience it, because that’s the only context in which morality makes any sense at all.
