For several generations, the Church has used a kind of shorthand in referring to mortal sin, for example, “X is a mortal sin.” The problem is that this general statement is an oversimplification. In order for the individual committing a particular act to be guilty of a mortal sin, three conditions are necessary: grave matter (the act must be intrinsically evil), full knowledge, and deliberate consent (CCC 1857).
The phrase “X is a mortal sin” gets tossed around on here with abandon. I observe two main discussions on the subject of mortal sin:
Armchair moralists declaring everything bad to be a mortal sin, out of hand, without knowledge of the circumstances or intention of an act (since moral acts are comprised of these things in addition to the “object” of the act)
People trying to find loopholes–if it’s a sin, but not a mortal sin, well, who cares, right?
Monsignor Pope very clearly and concisely puts to rest both forms of the discussion, neither of which is very helpful. All sin is bad. Not all sin is mortal.