To commit a mortal sin the Christian must: (1) have committed a grave sin; (2) have known its gravity; and (3) have been free to avoid offending God.
1. The offense in itself must be serious or grave, that is, something that has been forbidden by God under pain of losing His friendship.
2. The person who commits the sin must know what he is doing, and that what he is doing is a serious offense against God. (If, on the other hand, he does not know this but deliberately kept himself in ignorance about the matter, his ignorance is not excusable.)
3. There must be full consent of the will. A person who acts under any circumstance which would deprive him of the full exercise of his will would not be guilty of mortal sin.