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Moral culpability means how guilty a sinner is before God for having committed some sin. This moral responsibility is part of what's called a formal sin. The sinful act someone unwittingly commits, however, is called a material sin.
A person's subjective or imputed guilt in committing some objectively sinful act is not the same for everyone, owing to personal mitigating circumstances. These circumstances include a sinner's ignorance about the sinfulness of his action or his lack of free will in choosing to commit the sin.
Two people committing the same sin aren't punished equally according to Luke 12:47–48 where Our Lord attests, "That servant who knew the will of his lord ... and did not act according to his will, will be beaten with many stripes. But he that knew not, and did things worthy of stripes, shall be beaten with few stripes."
Culpa is Latin meaning fault or blameworthy. For complete culpability, a person must have sufficient awareness of the sinfulness of his action and give full consent of his free will when committing the sin. Habitual sin that causes a certain addiction may diminish a person's free will also called voluntariety. Lack of free will decreases one's culpability. Likewise, their ability to reason and the time given for deliberation affect a person's moral responsibility. A crime done in the heat of passion is viewed differently by the courts and by God as a crime done with deliberation or "in cold blood."
A person's guilt is also diminished when committing sins out of what's called invincible ignorance. This is ignorance that couldn't be overcome. Vincible or culpable ignorance, however, doesn't free one from guilt as the sinner could've and should've known better. A surgeon, for instance, is responsible for knowing basic surgery prior to operating. Likewise, a Catholic, who can't read a catechism, still has enough common sense from the natural law to know that murder is wrong. A person who doesn't want to know better has willful ignorance for which he's culpable.
Learn about what constitutes personal sin in season one of Church Militant's Premium show, Armor of God—Sin & Its Kind.