I have been a Protestant all my life and am considering converting. What is a mortal sin?
Mortal sin is a sin that is so serious it completely turns us from God. Venial sins only hurt our relationship with God; they do not completely turn us from God.
We see this reflected in our own human relationships. There are offenses that hurt our feelings and hurt our relationships, and then there are serious offenses that require an overt action on the part of the offender to repair the relationship.
From the Baltimore Catechism:
Q. 280. What is mortal sin?
A. Mortal sin is a grievous offense against the law of God.
Q. 281. Why is this sin called mortal?
A. This sin is called mortal because it deprives us of spiritual life, which is sanctifying grace, and brings everlasting death and damnation on the soul.
Q. 282. How many things are necessary to make a sin mortal?
A. To make a sin mortal, three things are necessary: a grievous matter, sufficient reflection, and full consent of the will.
Q. 283. What do we mean by “grievous matter” with regard to sin?
A. By “grievous matter” with regard to sin we mean that the thought, word or deed by which mortal sin is committed must be either very bad in itself or severely prohibited, and therefore sufficient to make a mortal sin if we deliberately yield to it.
Q. 287. How can we know what sins are considered mortal?
A. We can know what sins are considered mortal from Holy Scripture; from the teaching of the Church, and from the writings of the Fathers and Doctors of the Church.