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commit a mortal sin (please define this)

66. What is mortal sin?
Mortal sin is a grievous offense against the law of God.
Flee from sins as from the face of a serpent; for if thou comest near them, they will take hold of thee. (Ecclesiasticus 21:2)

67. Why is this sin called mortal?
This sin is called mortal, or deadly, because it deprives the sinner of sanctifying grace, the supernatural life of the soul.
Before man is life and death, good and evil; that which he shall choose shall be given him. (Ecclesiasticus 15:18)

68. Besides depriving the sinner of sanctifying grace, what else does mortal sin do to the soul?
Besides depriving the sinner of sanctifying grace, mortal sin makes the soul an enemy of God, takes away the merit of all its good actions, deprives it of the right to everlasting happiness in heaven, and makes it deserving of everlasting punishment in hell.
For the wages of sin is death; but the grace of God, life everlasting in Christ Jesus our Lord. (Romans 6:23)

69. What three things are necessary to make a sin mortal?
To make a sin mortal these three things are needed:
first, the thought, desire, word, action, or omission must be seriously wrong or considered seriously wrong;
second, the sinner, must be mindful of the serious wrong;
third, the sinner must fully consent to it.

70. What is venial sin?
Venial sin is a less serious offense against the law of God, which does not deprive the soul of sanctifying grace, and which can be pardoned even without sacramental confession.
Be ye therefore perfect, as also your heavenly Father is perfect. (Matthew 6:48)

"commit a mortal sin (please define this)"

I think everyone can agree that is a good definition of a mortal sin. Since the authors of the Baltimore Catechism cite Ecclesiasticus (also know as the book of Sirach,) which not all Christians accept as Scriptural, it's good to reference mortal sin from another source in Scripture:

"If any one sees his brother committing what is not a mortal sin, he will ask, and God will give him life for those whose sin is not mortal. There is sin which is mortal; I do not say that one is to pray for that. All wrongdoing is sin, but there is sin which is not mortal. (1 John 5: 16-17, RSV)

Here the Apostle John points out that there is sin which is mortal, and sin which is not, (what many of us refer to as venial.) Both are wrong and contrary to life in Christ, but there is definitely a more serious category.

I usually start by thinking of mortal sins as being sins which break the 10 Commandments. Worship of a false god, Adultery, Murder, theft, and bearing false witness.