Is the Catholic teaching regarding the definition of mortal sin de fide?

Question

The Catholic definition of mortal sin:

1. Its subject matter must be grave.
2. It must be committed with full knowledge (and awareness) of the sinful action and the gravity of the offense.
3. It must be committed with deliberate and complete consent

Is this definition De Fide (dogma)? Or are Catholics permitted to hold to a different definition of mortal sin, for example the Eastern Orthodox (and potentially Eastern Catholic) definition which states that a mortal sin is "any sin for which you do not repent"?

Answer 1

I don't know if there has been a solemn definition of the doctrine by a pope or council, but this definition is part of the deposit of faith. It is defined exactly as you state in the Catechism of the Catholic Church in point 1857, and the distinction between venial and mortal sin is also defined in point 1854.

Point 1854 states: "Sins are rightly evaluated according to their gravity. The distinction between mortal and venial sin, already evident in Scripture, became part of the tradition of the Church. It is corroborated by human experience."

This point states that the doctrine is present in Scripture and is part of the tradition of the Church.

If we understand dogma to be "a truth appertaining to faith or morals, revealed by God, transmitted from the Apostles in the Scriptures or by tradition, and proposed by the Church for the acceptance of the faithful." Then yes, this definition of mortal sin is dogmatic.

If by dogma we mean a solemn declaration from the Church, I would still argue that yes it is dogmatic since it is defined in the CCC which is the product of a Council.

The Council of Trent also discusses mortal sin, but I haven't been able to find a definition in it. Perhaps one of the earlier councils has a definition.

Answer 2

As Wtrmute mentioned, #2 & #3 are necessary for sin because sin is a bad human act, or as St. Augustine defined sin, "Sin is a word, deed, or desire against the eternal law." (Contra Faust. xxii). Since sin is a human act, the will is the subject of sin; there's no such thing as an involuntary sin.

Regarding #1, chapter 15 of the 6^th session of the Council of Trent, on justification, states that the sins listed in 1 Cor. 6:9-10 are mortal/lethal (i.e., always grave subject matter):

That, by every mortal sin, grace is lost, but not faith.

In opposition also to the subtle wits of certain men, who, by pleasing speeches and good words, seduce the hearts, of the innocent, [Rom. xvi. 18] it is to be maintained, that the received grace of Justification is lost, not only by infidelity whereby even faith itself is lost, but also by any other mortal sin [mortali peccato] whatever, though faith be not lost; thus defending the doctrine of the divine law, which excludes from the kingdom of God not only the unbelieving, but the faithful also [who are] fornicators, adulterers, effeminate [i.e., sodomites], liers with mankind [i.e., men who sleep with men], thieves, covetous, drunkards, railers, extortioners, [1 Cor. vi. 9, 10] and all others who commit deadly sins [letalia peccata]; from which, with the help of divine grace, they can refrain, and on account of which they are separated from the grace of Christ.

^{(Also, for more info on the division of sin into venial and mortal, see St. Thomas Aquinas's Summa Theologica I-II q. 88.)}