Probably daily across this country a few conversation go like this between young Catholics:

Catholic 1: Did you know pre-marital sex is a mortal sin?
Catholic 2: Well, did you know for a sin to be mortal you have to have full-knowledge and full-consent?
Catholic 1: I guess so, but does that mean it’s not a mortal sin to have pre-marital sex?
Catholic 2: Not if you didn’t know it.

Catholic 2 is a smarty-pants who takes St. Thomas out of context, but is still morally wrong. Here’s why: Even the new Catechism of the Catholic Church (CCC) admits that you don’t get a “pass” from God (so to speak) on ignorance of the moral law unless there’s absolutely no way you could have known the truth:

A human being must always obey the certain judgment of his conscience. If he were deliberately to act against it, he would condemn himself. Yet it can happen that moral conscience remains in ignorance and makes erroneous judgments about acts to be performed or already committed. This ignorance can often be imputed to personal responsibility. This is the case when a man “takes little trouble to find out what is true and good, or when conscience is by degrees almost blinded through the habit of committing sin.” In such cases, the person is culpable for the evil he commits. Ignorance of Christ and his Gospel, bad example given by others, enslavement to one’s passions, assertion of a mistaken notion of autonomy of conscience, rejection of the Church’s authority and her teaching, lack of conversion and of charity: these can be at the source of errors of judgment in moral conduct. If – on the contrary – the ignorance is invincible, or the moral subject is not responsible for his erroneous judgment, the evil committed by the person cannot be imputed to him. It remains no less an evil, a privation, a disorder. One must therefore work to correct the errors of moral conscience.—CCC 1790-1793.

That means you only get to claim invincible ignorance of the moral law of the Catholic Church if there’s no other way you could have, would have, should have known the truth.  Absent that, you don’t have true ignorance.  That means almost every grave sin is automatically a mortal sin, at least to anyone with access to the internet or a Catholic library or even access to a mildly-orthodox Catholic priest.

In other words, applied to the conversation at the top of this article, both Catholic 1 and Catholic 2 could have searched on the internet “What does the Catholic Church teach about pre-marital?” Granted, they’d have to wade through a few heretical answers, but eventually the truth would percolate to the top.  This means that because both Catholic 1 and Catholic 2 have access to the internet, both could only claim “vincible ignorance” if they chose to have pre-marital sex.  This means neither get a “pass” before God on their judgment if they chose to play games of “full knowledge and full consent of the will” while on earth on serious issues like pre-marital sex.  Thus, it’s very dangerous to play legalistic games with the God who sees every aspect of our heart and mind.

Of course, a pygmy in Papua New Guinea without the internet can truly claim “invincible ignorance.”  But that person needs to meet a missionary to teaching him the Gospel of Jesus Christ and baptize him in the first place. That is why you never read saints like St. Francis Xavier pushing legalistic games against the moral-law:  Such missionary saints always knew that Jesus and the law of God was the only way to the Blessed Trinity in heaven. 

Another aspect of darkness in the hearts the people who always harp about grave-sins never being mortal-sins until one has “full-knowledge” is that they make the mistake of assuming that the moral law of God is only intellectual, found in books, classrooms and online chats.  Think about that:  Why do legalistic white liberal Catholics always assume knowledge of God is only intellectual and not intuitional?

The Holy Spirit teaches us through St. Paul that one need not be Christian (or even Jewish as he is writing to the new converts in Rome) to know that certain unnatural sins are offensive to the one true God, even before one has any intellectual catechesis.  Why?  Because God already wrote His moral law on every human heart (cf Jer 31:33.)

St. Paul writes the following specifically about how certain pagan tribes fell into grave sin and how other certain pagan tribes fell into sins so serious (alphabet soup sins) that they marred even the natural law in their hearts and minds.  Notice that St. Paul reveals that not even pagans get a “pass” on the moral law for such abominable sins.  St. Paul even goes so far as to say they are “without excuse:”

For what can be known about God is plain to them, because God has shown it to them. For His invisible attributes, namely, His eternal power and divine nature, have been clearly perceived, ever since the creation of the world, in the things that have been made. So they are without excuse. For although they knew God, they did not honor Him as God or give thanks to him, but they became futile in their thinking, and their foolish hearts were darkened. Claiming to be wise, they became fools, and exchanged the glory of the immortal God for images resembling mortal man and birds and animals and creeping things.  Therefore God gave them up in the lusts of their hearts to impurity, to the dishonoring of their bodies among themselves, because they exchanged the truth about God for a lie and worshiped and served the creature rather than the Creator, who is blessed forever! Amen. For this reason God gave them up to dishonorable passions. For their women exchanged natural relations for those that are contrary to nature; and the men likewise gave up natural relations with women and were consumed with passion for one another, men committing shameless acts with men and receiving in themselves the due penalty for their error.—Rom 1:19-27.

All of this is to say that even before one has received solid intellectual catechesis, one still has received the law of God written on one’s heart.  Thus, one can never really claim ignorance.  Thus, grave sins are almost always mortal sins.