Conditions for mortal sin

[Vico]

Fr. John Hardon Modern Catholic Dictionary:
Grave Sin

The transgression of a divine law in a grievous matter with full knowledge and consent.

The matter may be serious either in itself (as blasphemy) or because of the circumstances (as striking one’s father or mother) or on account of its purpose (as telling a lie in order to destroy a person’s character). Sufficient knowledge of the serious nature of a sinful action is present if one is clearly conscious that the act is mortally sinful, say because the Scriptures or the Church identify certain acts as seriously offensive to God. It is enough that one knows that what one intends to do may be a mortal sin, but does it anyhow. Indifference to the laws of God is equivalent to disobeying them.

Full consent is present when one freely wills to commit an action although one clearly knows it is gravely sinful. No sin is committed if one does not will the deed, no matter how clear one’s knowledge may be. After all, the essence of sin is in the free will. Thus, too, a person does not sin who, with the best of will, cannot dispel obscene or blasphemous thoughts and desires, even though he or she well knows they are gravely sinful. The resolution to perform an action is not the same as the pleasure or satisfaction experienced in the emotions, nor the same as a compulsive idea, “I like the sin.” One sign of partial knowledge or not full consent would be the fact that a person does not complete an action when this can easily be done, or is so minded that the person would rather die than commit a grave sin.
catholicculture.org/culture/library/dictionary/index.cfm?id=33808

 

[underacloud]

• Full consent of the will (Again this is simple. If you know a particular act is a sin of grave matter and go ahead and do it anyway then you have committed a mortal sin).

No, this is not “simple”. You can know something to be grave matter, and you can go ahead and do it anyway, but there can be factors that may diminish culpability; factors that make the choice to sin something less than full consent.

As noted from the catechism:

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.

Also, this time with regard to the specific sin of masturbation (although the principles may carry across other sins):

*2352 … To form an equitable judgment about the subjects’ moral responsibility and to guide pastoral action, one must take into account the affective immaturity, force of acquired habit, conditions of anxiety or other psychological or social factors that lessen, if not even reduce to a minimum, moral culpability. *

I am not making this point to diminish the notion of mortal sin, but simply to balance the idea that full consent of the will is as easily understood as suggested. People who struggle with certain sins that are grave matter may or may not be committing mortal sin each time they do so; they need to make frequent confessions and to submit to their confessor’s guidance, but that guidance may include such direction as to not abstain from receiving Eucharist even though they commit such grave matter. Equally of course, a confessor may require confession prior to receiving; either way, the guidance of the confessor ought to be followed, but not as to excuse any such sins.

 

[Bookcat]

Blessed Pope John Paul II wrote of mortal and grave:
However, as we have already said, even in actual sin, when it is a case of grave (mortal) sin, man chooses himself in opposition to God. He chooses the creature in place of the Creator, and he rejects the Father’s love as did the prodigal son in the first phase of his foolish adventure. To a certain extent every human sin is an expression of that “mystery of iniquity” (2 Thess 2:7) which St. Augustine summarized in the words: amor sui usque ad contemptum Deiae, love of self to the point of contempt of God 3] .
vatican.va/holy_father/john_paul_ii/audiences/alpha/data/aud19861029en.html

and he importantly noted basically noted that one does not have to choose the mortal sin specifically to reject God or something (as some have thought) etc --but it can be “for whatever reason”. He does not have to intend to reject God etc in saying choosing to commit lust or murder etc.

 

[Bookcat]

Compendium issued by Pope Benedict XVI

1. When does one commit a mortal sin?

1855-1861
1874

One commits a mortal sin when there are simultaneously present: grave matter, full knowledge, and deliberate consent. This sin destroys charity in us, deprives us of sanctifying grace, and, if unrepented, leads us to the eternal death of hell. It can be forgiven in the ordinary way by means of the sacraments of Baptism and of Penance or Reconciliation.

1. When does one commit a venial sin?

1862-1864
1875

One commits a venial sin, which is essentially different from a mortal sin, when the matter involved is less serious or, even if it is grave, when full knowledge or complete consent are absent. Venial sin does not break the covenant with God but it weakens charity and manifests a disordered affection for created goods. It impedes the progress of a soul in the exercise of the virtues and in the practice of moral good. It merits temporal punishment which purifies.

vatican.va/archive/compendium_ccc/documents/archive_2005_compendium-ccc_en.html