Trying to understand when I have committed a Mortal Sin. "Knowledge, or firm belief that an act is seriously wrong"


I am reading through one of those big grave matter lists.

This one to be specific.

“Knowledge, or firm belief that an act is seriously wrong”

It’s the “firm belief” part that is getting me.

For an example. I firm belief in me that killing someone is wrong, and is a sin. I feel it in my conscience. However, I do not feel that having sex with my fiancee is a mortal sin, I do not have a firm belief in it. I am open to it being disclosed to my conscience and all, but right now it just doesn’t feel that way to me.

So my question is, when are we actually ticking the box for the second requirement?

I can understand that if I have that sting in me, that voice that is saying, ‘this is wrong, don’t do it’ and I go ahead an do it anyway that this is most likely a mortal sin, if it is a grave matter.

But what of those things that that voice is not operating, and when I do try and consider, is what I am doing wrong, and it just doesn’t seem to check as being wrong. Am I still in mortal sin?

Help with this would be of great benefit. Many thanks.



Yes, the requirement is met.

Just because you don’t have the “firm belief” that premarital sex is wrong, you already know it to be so, because the Church has taught you this, and to whom you owe assent in matters of faith and morals. So this does not excuse you. It is, in fact, grave matter, and you know it. If it did, we could simply pick our favourite sin and say I don’t “firmly believe” this. It doesn’t work that way. While we are obligated to always follow our consciences, we at the same time have the obligation to form them properly.

The “firm belief” part here is when someone actually, firmly believes that, for example, stealing a penny from a billionaire is grave matter, and despite that, still does it. The matter itself is not objectively grave, but the person had this hardness of heart that turned him away from God.


Reducing the whole thing to a checklists seems pretty mechanical to me.

That site is pretty off the wall too. Among other oddities it claims that the rapture is sound theology.

I wouldn’t trust anything at that site.



The operative word here is “OR”. If you have knowledge that something is a sin, usually though your knowledge of Scripture and the teachings of the Church, it IS a sin. If, in absence of that clear knowledge, you have a firm belief that something is wrong, then to commit the act would be a sin.

In your example, since the Bible and the teachings of the Church clearly prohibit fornication, it is a sin whether you have a firm belief in that teaching or not. If your conscience doesn’t prick you when you violate Church teaching, you may not have a properly formed conscience.


:thumbsup: That list is dangerous, especially to anyone inclined to scrupulosity. I suggest reading the Catechism of the Catholic Church paras 1854 - 1876. Para 1859 explains what a mortal sin is. The search engine doesn’t seem to be working on it at the moment so you may have to keep jumping sections to find it.


When in doubt, check the Catechism.

The Catechism of the Catholic church states: 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.”(CCC 1857)

The Catechism also states: 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. (CCC 1859)

Note that, according to the Catechism of the Catholic Church, personal belief in whether or not an act is sinful has no effect on the actual gravity of a sin. Regardless of how a person feels, if he knows an act is considered to be a mortal sin by the Church, and he chooses to engage in that act anyway, then that person has committed a mortal sin.

Here’s the link to the catechism if you’re interested:


Try to reframe the question. Instead of “How much can I get away with and still go to Heaven?” (which is actually a T-shirt I saw somewhere :smiley: ) try to cultivate virtue. Ask "How can I grow in purity and chastity, respecting my and my beloved’s God-given gift of sexuality? How can we, hopefully as a couple, come to a deeper understanding of the awesomeness of the unity that sex is meant to express when used in its proper context of marriage? That awesomeness includes nothing less than the possibility of co-creating a new human life with God! :slight_smile: And it cements the husband-wife relationship. Finally, it reflects the marital relationship Christ has with His Bride the Church.

There are even biological and emotional bonding forces at work - which is probably why the desire to have sex is feeling so strong. You want to make sure you can live out a true bond and commitment in reality, not just wishful thinking. Study Theology of the Body For Beginners, or Christopher West’s The Good News About Sex and Marriage, check out the resources on There is so much good, positive, and helpful information out there, and it’s clear in morals while realistic about the temptations people face, and gives constructive ways to deal with those. Good luck, and I’ll be praying for the both of you! :getholy:


Thanks for your replies everyone. Ok I get the picture, if I know what the Church, and Scriptures say are a sin, I am in full knowledge?

Would this be correct?

Is there a good list of Catholic grave matters, and venial sins?

In relation to sex within marriage. What about this piece of Scripture.

“Now concerning the matters about which you wrote: “It is good for a man not to have sexual relations with a woman.” 2 But because of the temptation to sexual immorality, each man should have his own wife and each woman her own husband. 3 The husband should give to his wife her conjugal rights, and likewise the wife to her husband. 4 For the wife does not have authority over her own body, but the husband does. Likewise the husband does not have authority over his own body, but the wife does. 5 Do not deprive one another, except perhaps by agreement for a limited time, that you may devote yourselves to prayer; but then come together again, so that Satan may not tempt you because of your lack of self-control.”

The reason it seems, in this piece of scripture for being married (in addition to children, and companionship) is to have sex! “But because of the temptation to sexual immorality, each man should have his own wife and each woman her own husband.”

And goes onto say each of them for the reason of temptation of fornication should give each other to each other, whenever one of them asks of it.

As far as I can see this is a clear passage that says, have sex, to have sex! Sex is not just for children, but also to ward away temptation of satan.

Do ye not think so?


If you don’t have a copy of the Catechism, there is another section that is relevant to your question here.
ccc 1735 Imputability and responsibility for an action can be diminished or even nullified by ignorance, inadvertence, duress, fear, habit, inordinate attachments, and other psychological or social factors.**

This is a really vague and general statement that refers to whether you have complete freedom in a situation. So, something like fornication (having relations with a girlfriend) may be objectively a mortal sin, but a person committing it may have diminished responsibility for it.

I’ve sought guidance on this paragraph of the Catechism, what does it mean. the answers are mostly like those preceding this post, it’s wrong, it’s wrong, it’s wrong. I believe that this paragraph is not as legal sounding as it may be, rather , it’s about God’s mercy. There are circumstances, perhaps many of them, which diminish a person’s responsibility for some action. But, I have found no definitive explanation of it.

And this paragraph does not even list all the reasons for which our responsibility might be diminished. 1735 is based on the teaching of the Bible. For example, on the cross, Jesus prays, Father forgive them, for they know not what they do. <-- ignorance. These people think they know what they’re doing, but they really don’t, and Christ prays for their forgiveness.

In the Old Testament, there’s an example of a slave woman who is forced into a sexual encounter against her will – she is not guilty of it.

It isn’t right for me to delve further into your personal matter that you described. But, as Fr. John Riccardo, radio priest in the Archdiocese of Detroit says, God is not waiting up there to drop an anvil on our heads, if we make a mistake. Others have made more pointed remarks above.

We should, though, have a profound respect for God’s commandments. It’s not enough to know them, we have to live them. And, we have to know that there are consequences to our sins, not just in eternity, but here and now. Having unmarried sexual relations can have a lot of consequences, including leading us further into sin.

Scripture, essentially, teaches us to control our passions. We should learn this lesson.


I read a lot of Jewish commentaries (which is permitted explicitly by the Church).

There’s a book called*** The 6 Constant Mitzvos*** [commandments]. Now, be forewarned, this book has a lot of Hebrew words in it. You’d have to look them up online in an online Jewish dictionary. But, you would easily get the drift of this book.

What I like about this book is the fervor for obeying God’s law. That might impress you if you read the Catechism of the Catholic Church, but I think the Jewish fervor for the commandments starts right in the Bible, and Psalm 119. If I remember correctly, there’s a verse that goes 'Thy word O LORD is a lamp for my feet and a light for my path." and “Thy word have I hid in my heart that I might not sin against Thee.” Remember though, this book is the Jewish tradition.

Another one in that tradition is Love Your Neighbor and Yourself by Elliot N. Dorff. It’s a modern Jewish perspective, not all of which agrees with Catholic teaching, and arguably does not entirely agree with the Bible either, but that excitement and fervor for loving and serving God is captured there too.


Forgive me, but I am curious as to why you are so concerned about what the Church teaches about this when you self identify as “advaita/being”? I am pretty well read in religion and don’t have a clue what that means. Maybe explaining that will help us help you?

It is grave matter because of the grave obligation to follow one’s conscience, even if your conscience is wrong. Does this make sense to you?


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