(Latin: mors, death)
A grievous offense against the law of God.
This sin is called mortal because it deprives us of supernatural life and brings damnation and death of the soul.
Three conditions are necessary for a mortal sin: gravity of matter, sufficient reflection, and full consent of the will.
The gravity of matter is determined by Holy Scripture, by the definitions of the Church, by the testimony of the Fathers, Doctors, and theologians, by the universal belief of the faithful, and by reason enlightened by faith.
Mortal sin is a revolt against God, supreme Lord, contempt of His adorable majesty, an act of monstrous ingratitudc.
It is an offense against Christ who redeemed us, and against the Holy. Ghost who sanctifies us.
It deprives one of sanctifying grace and thus prevents one from acquiring merit or sharing in the satisfying merits of the Church.
It tarnishes the soul, and causes remorse of conscience, an inclination to evil, darkening of the intellect, weakening of the will.
It deprives one of the right to heaven, and entails penalties, some of which are incurred in this life, and the loss of God forever as well as eternal punishment.
New Catholic Dictionary