How do we know if a sin is grave?

Hello! I was wondering about how we know if a sin is grave or not, when talking about mortal sins and such. I mean obviously the big ones like rape, murder, so on and so forth but what exactly makes a mortal sin grave? How does one call a sin a mortal sin without knowing exactly what the action grave. Because if we knowingly commit a sin, but honestly did not know it was grave enough to be a mortal sin(and I’m playing the devils advocate just for the sake of the question) then how are we able to consciously identify that mortal sin to mortal and confess it to God and seek forgiveness? If that makes any sense at all… Sorry, Thank you!

Please consider checking this out… newadvent.org/cathen/14004b.htm#III

The Catechism says:
1858 Grave matter is specified by the Ten Commandments, corresponding to the answer of Jesus to the rich young man: "Do not kill, Do not commit adultery, Do not steal, Do not bear false witness, Do not defraud, Honor your father and your mother."132 The gravity of sins is more or less great: murder is graver than theft. One must also take into account who is wronged: violence against parents is in itself graver than violence against a stranger.

In each circumstance of an individual sin, what constitutes a grave matter, is more complex than one paragraph from the Catechism.

Someone with a moral theology degree would do a much better job than me explaining this, but i understand people come here looking for quick answers and will not necessarily read a long document provided thru a link.

Here is more info from a web page called "the Way of the Lord Jesus’:*

Consider a man who would like a morning paper. Checking for change, he discovers that he lacks the coins necessary to purchase a paper from a vending box. However, he notices that the box is not latched tightly. The thought occurs: “I could take a paper without paying for it, but that would be wrong. However, the newspaper company will not be seriously hurt if I take a paper and close the box, so that subsequent customers will pay.” He hesitates momentarily, realizing the wrongness of the act, but is inclined to choose to do it anyway. A quick look about assures him there is no one to notice his pilferage. He filches the paper.

In a case like this, one might suppose that he did not reflect sufficiently and consent fully. However, the supposition of the example precisely is that he did. One also might suppose that he entertains some thoughts which could justify the act—for example, that he would pay double next time or that on some previous occasion the box has taken his coins without opening. But let us suppose he had no such thoughts.

Catholic moralists and the faithful in general would agree that, despite the sufficient reflection and full consent in this case, the act was not a mortal sin. What the man did is light matter.

4. A few theologians have held that of itself every kind of immorality would be grave matter, but God by a merciful fiat simply decrees that many sins people are likely to commit will not be mortal. This view is unsatisfactory, since it presupposes a legalistic conception of the relationship between moral action and one’s share in divine life. If God determines by fiat which immoralities remain mortal sins, there seems to be no intrinsic connection between living an upright life and remaining in his friendship. The requirement to avoid mortal sin becomes an arbitrary test.

I hope this helps, distinguishing grave matter and mortal sin is a delicate subject.*

Also
CCC 1859 “Mortal sin requires full knowledge and complete consent. It presupposes knowledge of the sin character of the act, of its opposition to Gods law… …Feigned ignorance and hardness of heart do not diminish but rather increase the voluntary character of a sin”

Just dont fall into the trap of convincing yourself "Oh I didn’t know , wink wink nudge nudge " Because as you can see … That in itself can be an issue

This was perfect, thank you so much for taking the time, it is very much appreciated!

This is good to know, I mean I figured convincing yourself a sin is not a sin just proves that well, it is a sin! But it’s always good to hear these things from others because it just solidifies it in my head that you can’t pretend with God, he always knows! Thanks!

The other thing you can do is to take this to Father in confession after working through a good examination of conscience.

It’s always good to take venial sins to Confession, too. Regular Confession, perhaps monthly, is a good practice to take up even if one has no mortal sins to confess.

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