Following St. Alfonso Liguori, most priests have been taught that three conditions on a given act must obtain in order for the act to count as a "mortal sin": grave matter, full knowledge, and full consent. 'Grave matter' refers to the objective moral quality of the act: a wrongful act counts as grave matter just in case it is intrinsically and thus objectively wrong, whether or not the sinner knows that. What counts as grave matter is typically easy to identify given the teaching of the Church and the wisdom of the race. Full knowledge is clear knowledge that the matter is grave; full consent is the uncoerced choice to perform the act in light of full knowledge. Sounds reasonable; and rightly understood, it is. But there is a difficulty.
Some theologians have held that full consent is rarely given to acts known by the agent to constitute grave matter. The argument goes roughly like this. If and when one knows the matter to be grave, then one knows the best reason not to do the corresponding deed; people do not desire what they know to be evil, but only what they believe, rightly or wrongly, to be good; so it seems likely that, when they do something of a sort they know to be objectively wrong, they are doing so in virtue of some influence on the will which diminishes their freedom and thus precludes full consent. Hence mortal sin is rare. Arguments like that motivate such theories as that of the "fundamental option," according to which mortal sin occurs not so much with the commission of this or that objectively wrong act, but only with a clear and explicit choice against God—a choice that might manifest itself in such acts, but not itself be constituted by the sorts of act they are in themselves.
The philosophical difficulty with the fundamental-option theory (FOT) is fairly obvious and decisive. If the more traditional view is correct, then any act which is intrinsically and thus objectively wrong just is a sort of act which, if done with full knowledge and consent, entails the exercise of a fundamental option against God. That is precisely why, in order to avoid begging the question, FOT advocates need the premise that full consent to such acts is rarely given. But if full consent is rarely given, inasmuch as people do not desire what they know to be evil, then a fundamental option against God is probably the least likely candidate for an option freely undertaken. Nothing could be more irrational than standing before God, knowing him and his will according to classical theism, and saying "No" to him. So if the moral psychology on which FOT relies were correct, then the fundamental option against God seems too irrational to elicit full consent, and hence cannot be said to be exercised at all. This is why, in my experience, most FO theorists end up as universalists. Perhaps many were so from the start. They just can't believe that, at the end of the day, there are people who defy God freely and knowingly. Hell is too irrational a choice for anybody to be held accountable for making it. There must be some exculpating factor that would allow escape.
Since, as John Paul II pointed out, FOT is incompatible with the definitive teaching of the Church, we need not linger over it further. My concern is with the premise that full consent to what is known to be grave matter is rarely given. One logical consequence of such a premise is that, the better one knows the correct moral theology, the less capable one is of mortal sin. That consequence is manifestly untrue, and I trust I don't need to cite examples. Sound moral theology can help arm one against sin, and sometimes does help; but it's by no means enough; what counts for more is prayer and character, which can be facilitated by moral theology but can also develop without it. That's a logical reductio of the premise that full consent is rarely given to what's known to be grave matter. And then there's experience to cite.
There's a crucial fact usually overlooked by those who uncritically apply the Liguorian conditions: people can be and sometimes are culpable for lacking full knowledge of the wrongfulness of what they do, or propose to do. Countless are the cases when people can and ought to know better than they do, but they don't because at some level they have chosen not to. Whole societies can be swept up in that: e.g., Germany in the late 1930s, or Rwanda in the mid-90s. Thus, even when the Liguorian conditions for full knowledge and consent are lacking, there can be and are cases where full consent has been culpably withheld from the task of acquiring, or maintaining, full knowledge. Thus the sinner can be just as guilty as they would be if the conditions were clearly met at the time the obvious sin is actually committed. Rationalizations abound, and by no means are they all involuntary.
In my observation, however, many confessors and moral theologians fail to notice or give credence to that fact. They don't believe it worth asking whether the sinner can be guilty at some operative level below the one on which the sinner consciously operates at the time the sin is committed. Or, if it does occur to them to ask such a thing, they are reluctant to bring it up with the sinner. This phenomenon is very common in the area of sexual sin. A lot of Catholics fornicate and contracept without believing such things are wrong; some do so while having had every opportunity to learn the objective truth, which they prefer to rationalize away as a matter of private opinion. Faced with that, some confessors see nothing to be gained by insisting on the truth, even by presenting arguments for it. For in many such cases, the process by which lust commands the will, and with it the intellect, seems too far advanced to allow for true repentance in light of full knowledge. That is why, as I said in a post earlier this week, some priests fear raising people to a level of knowledge that would make them more culpable for rejecting the truth than they would be otherwise. Better to leave such folks ignorant while hoping that they learn something in the school of hard knocks. For the time being, they just can't handle the truth.
Of course there are many other reasons for soft-pedaling, if not altogether ignoring, the full truth. Truth that can't be handled is unpopular; those who press such truths usually get into trouble. Priests like that often get into trouble with their local chancery, and soon find their hopes for desirable assignments quashed. As for laymen, I can't tell you how many times I've been dismissed as a neurotic, or as one dead before his time, for choosing celibacy as a spiritual path after two divorces, even though I'm not required to make that choice. Most of the people I deal with day-to-day assume that I ought to fornicate, because I can—if only before I marry for a third time, cavalierly expecting hope to triumph over experience. Many of the people who make that assumption are, I'm ashamed to admit, Catholics. All I can say after rolling my eyes is that we live in a society that has lost its moorings about sex, and that there's more of society in the Church than one would like to see.
Personally, I'd feel much better about life if I had one-thousandth the opportunity for the seminary that I have for fornication. That the ratio is even less auspicious than that is a truth I can't quite handle yet.