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This is actually not a speculative question! :) An act committed with mere knowledge is not a mortal sin--it must be full knowledge. So, what is the difference between mere knowledge and the full knowledge required for a mortal sin? Where's the cutoff?

I don't know if this needs to be a second question--what's the difference between mere consent, which does not result in a mortal sin, and complete consent, which does? What's the cutoff?

The Catechism gives a partial answer: 1862 One commits venial sin when. . .he disobeys the moral law in a grave matter, but without full knowledge or without complete consent.

But it doesn't give a cutoff point for knowledge or consent. What's the cutoff? When does knowledge become the "full knowledge" and consent "complete consent" required for mortal sin? And how could I be absolutely sure I crossed that line?

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  • Please answer in light of Amoris Laetitia 301-306: Hence it is 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”,or be in a concrete situation which does not allow him or her to act differently and decide otherwise without further sin. As the Synod Fathers put it, “factors may exist which limit the ability to make a decision”
    – Ashpenaz
    Commented May 11, 2022 at 17:04
  • It takes more than mere knowledge and mere consent to commit a mortal sin--I can know something is wrong, and I can consent to doing it, and that's sin, but not a mortal sin. I have to have "full" knowledge and "complete" consent to commit a mortal sin. So, my question is--where's the cutoff? When do I move from having knowledge to having "full knowledge?" When do I move from having consent to "complete" consent?
    – Ashpenaz
    Commented May 11, 2022 at 19:05
  • related: cf. the last quotation of § "Moral certititude" of this answer
    – Geremia
    Commented May 11, 2022 at 21:36
  • How does moral certainty relate to full knowledge? They are both Catholic terms, but are they about the same subject?
    – Ashpenaz
    Commented May 11, 2022 at 21:44
  • Moral certitude regards judgments (in your case: judging whether a sin is mortal or venial).
    – Geremia
    Commented May 12, 2022 at 18:27

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St Thomas says, in the Prima Secundae, QQ. 76 a. 4

Since every sin is voluntary, ignorance can diminish sin, in so far as it diminishes its voluntariness; and if it does not render it less voluntary, it nowise alleviates the sin.

In other words, if knowing more about a sin would not have stopped you from committing X mortal sin, the ignorance you did have does not diminish the guilt of that sin.

Moreover, "ignorance which is not the cause of the sin being committed, but is concomitant with it, neither diminishes nor increases the sin."

Therefore sin cannot be alleviated by any ignorance, but only by such as is a cause of the sin being committed, and yet does not excuse from the sin altogether.

Ignorance "which is a cause" is the kind of ignorance I noted above. Ignorance can be a cause of sin insofar as it is a lack of knowledge that would prevent one from committing the sin. This depends on the person and circumstances, as he notes in Article 1:

If a man's will be so disposed that he would not be restrained from the act of patricide, even though he recognized his father, his ignorance about his father is not the cause of his committing the sin, but is concomitant with the sin: wherefore such a man sins, not "through ignorance" but "in ignorance," as the Philosopher states (Ethic. iii, 1).

"Now," he says, "it happens sometimes that such like ignorance is directly and essentially voluntary, as when a man is purposely ignorant that he may sin more freely," which makes the sin more voluntary and more sinful. Other kinds of ignorance may be indirect such as being unwilling to study as hard as one should, resulting in ignorance which causes some sin.

In sum, the question that must be asked is "would this knowledge have prevented the person from sinning?" The cutoff is whether the person would have still committed that sin with the knowledge he was missing.

This is basically just restated by the Catechism and by AL 301-302, with some possible examples. Part of why there just isn't a single hard-and-fast set of rules that apply always to everyone is because it requires reflection and discernment, sometimes especially by a confessor. Hence questions like these can easily veer into pastoral territory, because they are pastoral by nature when it comes to individuals!

"And how could I be absolutely sure I crossed that line?" By forming your conscience well and simply asking your confessor if in doubt about anything. Just about every online resource dealing with this ends in "when in doubt, ask your confessor!" for the reason I just stated.


Why is complete consent listed as a requirement for an act to be a mortal sin? was answered and that seems to answer your question on consent. If not you may want to ask a separate question.

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