What is the difference between a mortal sin and a venial sin?

This question is for a priest to answer, please. Thank you.

Baltimore Catechism:

Q. What is mortal sin?

A. Mortal sin is a grievous offense against the law of God.

Q. 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. How many things are necessary to make a sin mortal?

A. To make a sin mortal, three things are necessary: 1.a grievous matter, sufficient reflection, and full consent of the will.

Q. What is venial sin?

A. Venial sin is a slight offense against the law of God in matters of less importance, or in matters of great importance it is an offense committed without sufficient reflection or full consent of the will.

Q. Can we always distinguish venial from mortal sin?

A. We cannot always distinguish venial from mortal sin, and in such cases we must leave the decision to our confessor.

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