As far as I know, mortal sin constitutes 3 things, and they must be present for mortal sin to exist. These 3 things are:
(1) Grave matter (Something that is considered to be a very serious sin)
(2) Full Knowledge (Knowing that what you are committing is a grave sin)
(3) Full Consent of the Will (Agreeing to do what you're doing in a right frame of mind, unbarred by any addiction or mental disorder)
The way some N.O. Catholics (or CAF types) speak of mortal sin, it's almost as if it's virtually impossible for anyone to commit. It seems as though while grave matter is always present, the other 2 things go out the window very quickly. It's like atheists cannot be held culpable for their actions because they have not "known" God. Fornicators or "sex addicts" cannot be held culpable because they are not giving their consent to it. They're just addicted to sex! (sure..)
If 2 of the 3 conditions are missing (and it's usually the latter 2), then would that not make the matter a grave sin? If so, what is a "grave sin" and how does it differ from a mortal sin? There's no way I'm convinced that an act like abortion (one that is willed by the mother) is only a venial sin, even if someone doesn't "know" that Christianity condemns it as murder. The same holds true for atheists. Just because they don't "know" that their really is a God, that doesn't mean they aren't aware of what denying His existence really means.
Help me out here.
All three conditions have to be present at the time of commission in order for the sin to be mortal. Technically even two conditions could be present, however, if one is lacking, then the sin is not mortal.
I always found that St. Pius X's Catechism has the best definition of mortal sin.
From the Catechism of St. Pius X, "The Main Kinds of Sin," Question 9-10:
Q: What injury does mortal sin do the soul?
A: (1) Mortal sin deprives the soul of grace and of the friendship of God; (2) It makes it lose Heaven; (3) It deprives it of merits already acquired, and renders it incapable of acquiring new merits; (4) It makes it the slave of the devil; (5) It makes it deserve hell as well as the chastisements of this life.
Q: Besides grave matter, what is required to constitute a mortal sin?
A: To constitute a mortal sin, besides grave matter there is also required full consciousness of the gravity of the matter, along with the deliberate will to commit the sin.
I understand full consciousness to be another way of saying sufficient reflection.
Many times when I've done my examination of conscience I have to honestly ask myself it I did in fact ponder the sin over in my mind before I committed it.
In any case, I find it best to confess everything irregardless.