And who is the arbiter of what is mortal and what is venial? What standard or authority is used?
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.
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God is the ultimate authority, of course. Below that the Church can classify actions, deeds, words, or omissions that are objectively mortal sins. But subjectively, this is a matter between a person's conscience, a confessor (who acts to clarify our thinking), and God.
We can say for a fact that if you do X, that it is a mortal sin. We can not say that when X has occured, the actor was mindful that this was a serious wrong or had fully consented to the action. That is not something you can codify. This is the difference between a legal code and a justice system.
The Church doesn't pick words haphazardly. And we are invited to examine our own actions often for the times we have failed to follow his commandments and love one another and to seek forgiveness.
SD