"Full Knowledge" Element for Mortal Sin?


CCC 1857 says: For a sin to be mortal, three conditions must together be met: “Mortal sin is sin whose object is grave matter and which is also committed with full knowledge and deliberate consent.”

What constitutes “full knowledge”?


  1. Assume that I am a baptized Catholic.
  2. Assume that I know that the Church teaches that fornication is gravely sinful.
  3. Assume further that I simply do not believe that the Church’s teaching is true.
  4. Assume also my lack of belief is sincere, even if misguided.
  5. Finally, assume that, following my conscience, I with deliberate consent engage in acts that I know the Church says are gravely sinful.

Does my lack of belief, or failure to appreciate the reasons for the prohibition against fornication, equate to lack of “full knowledge” so that I remain in a state of grace and have not committed a mortal sin?


I would say no. As long as you are aware that the Church teaches that an action is wrong, you have full knowledge.

Simply not believing that it is wrong can not be an out. The greatest sinner in history, who killed millions and millions, believed they were acting for the greater good. That can’t excuse their sins.

God Bless


You have full knowledge because you fully know that the Church teaches the act to be gravely sinful but choose to do it anyway.


Hi 2PT3,

To me, these two statements are somewhat problematic. If a person believes the Catholic Church is the true faith, but believes that she is wrong on certain teachings, then how can that person say they believe the Catholic Church is the true faith?

Because of this, I would hold that knowing the Catholic Church teaches that something is gravely wrong suffices as “full knowledge”. Assuming one is Catholic and at least attempting to be a practicing one, then they have full knowledge.

It’s also important to note that there is vincible and invincible ignorance. So if a person was ignorant of the teaching, but out of negligence, then they are culpable for their ignorance.

Hope that helps!


What people believe or don’t believe has little or nothing to do with actual Truth.
Wanting to believe something is true, or not true, is a big problem in this secular age as it is like walking down the tracks shouting “I don’t believe in trains”. Not going to make a lot of difference when the train gets there!


So, what about where Amoris Laetitia teaches that my unwillingness to accept Church teaching, even when I “know full well the rule”, may excuse me from mortal sin due to lack of “full knowledge”? See, AL. par. 301: “Hence it can no longer simply be said that all those in any ‘irregular’ situation are living in a state of mortal sin and are deprived of sanctifying grace. More is involved here than mere ignorance of the rule. A subject may know full well the rule, yet have great difficulty in understanding ‘its inherent values"…"

In article “defending” Amoris Laetitia in *L’Osservatore Romano *(the Vatican’s official newspaper) on July 19, the following statement was made: “For a sin to be mortal, two other conditions are necessary beyond grave matter. It is also necessary that there be full knowledge of the evil of the act committed. If one is convinced in conscience that the act is not (gravely) evil, the action will be materially evil but not imputed to the person as a mortal sin.”
Is my saying; “I know full well the rule, but I don’t understand it or don’t believe it; therefore, I reject it” really enough to excuse me?



It is not that simple.


1859 Mortal sin requires full knowledge and complete consent. It presupposes knowledge of the sinful character of the act, of its opposition to God’s law. It also implies a consent sufficiently deliberate to be a personal choice. Feigned ignorance and hardness of heart do not diminish, but rather increase, the voluntary character of a sin.

1860 Unintentional ignorance can diminish or even remove the imputability of a grave offense. But no one is deemed to be ignorant of the principles of the moral law, which are written in the conscience of every man. The promptings of feelings and passions can also diminish the voluntary and free character of the offense, as can external pressures or pathological disorders. Sin committed through malice, by deliberate choice of evil, is the gravest.



In addition to that, we have the problem that obstinate post-baptismal refusal to accept the Church’s doctrine is itself a sin. It seems that voluntary doubt, incredulity, and heresy may come into play here. Surely, one of these sins cannot form the basis for my exculpation from another mortal sin for “lack of knowledge”?

2088 The first commandment requires us to nourish and protect our faith with prudence and vigilance, and to reject everything that is opposed to it. There are various ways of sinning against faith.

Voluntary doubt about the faith disregards or refuses to hold as true what God has revealed and the Church proposes for belief. Involuntary doubt refers to hesitation in believing, difficulty in overcoming objections connected with the faith, or also anxiety aroused by its obscurity. If deliberately cultivated doubt can lead to spiritual blindness.

2089 Incredulity is the neglect of revealed truth or the willful refusal to assent to it. "Heresy is the obstinate post-baptismal denial of some truth which must be believed with divine and catholic faith, or it is likewise an obstinate doubt concerning the same; **apostasy **is the total repudiation of the Christian faith; schism is the refusal of submission to the Roman Pontiff or of communion with the members of the Church subject to him."11



By your own statement number 2 “I know that the Church teaches that fornication is gravely sinful.” this would constitute full knowledge. I have seen too many like, Richard McBrien, who would lie to you and deceive you that the Church is teaching that “full knowledge” means you have the knowledge of God. It simply means do you know what the Church teaches and obviously you do.

What if you did not list No.2. You would still be culpable for mortal sin as, Bookcat cited CCC 1860 which is often intentionally left out.

CCC 1860 “…But** no one is deemed to be ignorant of the principles of the moral law,** which are written in the conscience of every man. …”


Yes, this was the point of my asking the question. AL 301 seems to posit that mere disbelief is “lack of full knowledge” that can exculpate one from mortal sin. If that is true, then no one is ever guilty of mortal sin - no matter what the Church says is sinful - if they sincerely disbelieve what they “know full well” the Church teaches. That seems flat wrong.


There is a difference between disbelief that arises from the lack of hearing the Gospel …from lack of evangelization or from all sorts of “mis-hearing” or bad formation etc …and that which is rather willful and the persons fault.

Each case must be judged - and in the end God is the final judge of each person.



Excellent Post.


OK, maybe fornicating may not be personally mortally sinful for you if you sincerely don’t get it.

But that doesn’t mean you are not engaging in acts that the Church regards as objectively disordered.

Are you prepared to try and live a non-fornicating lifestyle (even though you see nothing wrong with it) simply because its important to God that you do?

You also have an ongoing duty to seek counsel and make sure you really understand why the Church teaches this.
It may be helpful for you to explain here why you think the Church teaches this?

Strictly speaking you may not receive public communion either.


If you know the Church teaches that an action is of grave (serious) matter then that satisfies the “full knowledge” condition. Understanding is preferable but not necessary.


No, it does not.

If you sincerely believe that an act is moral, even though you know that the Church says it is a grave sin, you do not have full knowledge. The reason is that each person is judged by their own conscience.

Full knowledge is the full realization that the act is gravely immoral, according to one’s own sincere understanding of morality. And that is why invincible ignorance can make an objective mortal sin not also an actual mortal sin.

We can see the application of this principle in a case where a person sometimes even outwardly rejects the Church, and yet remains in the state of grace by invincible ignorance.

If thistle’s view were correct, then everyone who “knows” about Christianity and Catholicism, and yet refuses to convert, would be condemned to Hell due to actual mortal sin. But such a view is incompatible with Catholic teaching on salvation.


That is not correct. Please show me a Church document to support your claim.

If Ron Conte’s view were correct then nobody in the world could commit a mortal sin!


Unfortunately, many persons commit actual mortal sin. They realize that a particular act is gravely immoral, according to their own conscience, and yet they choose the act with full consent.

Can a person know about the Catholic Church, then outwardly reject Her, and yet still be saved? Yes, if the rejection is not an actual mortal sin, due to a lack of full knowledge. The person knows what the Church teaches, but his conscience errs by deciding that the Catholic faith is not the truest form of religion. This dispute about actual mortal sin and full knowledge is really about salvation. Who is saved? And are we judged by conscience, or simply by our compliance to objective moral norms?


Yes that can be correct.

It depends though somewhat one what one means.

For example it also be the case where a person - say a non Christian - hears that the Church says such and such is serious -but in their invincible ignorance - their conscience judges it is not serious - through no fault of their own.




Correct and the same holds for Catholics.

Does that mean they are not really Catholics?
In some sense I suppose it means they are not fully formed Catholics.
Yet if they are baptised and practising and in every otherwise conforming…that seems enough to legitimately be called Catholic.

Anyways, we are saved by sanctifying grace not by being “Catholic”.
God can make Catholics out of the very stones I believe.

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