There are three conditions required for a person to be guilty of a mortal sin: (1) the act or omission must be a grievous error; (2) you must believe that it is a mortal sin; and (3) you must will to do it.
Virtual all Catholics become skilled canon law scholars some time in their teen age years and become pretty skilled at rationalizing the sins that they confess when going to Confession. It isn't that difficult to manipulate the old conscience and turn them into uncontrollable addictions, genetically programmed instincts, cultural norms, harmless, etc. Thus a lot gets glossed over or totally omitted in the confessional.
Cathy of Alex, whose patroness, St Catherine of Alexandria is the patron saint of librarians, Cathy's occupation, blogging at The Recovering Dissident Catholic, has a great post today, niftily written, dealing with our propensities for sliding over our weaknesses, venial and mortal.
"...KNOWING the source material is the number one weapon of faithful Catholics. You don't have to have a degree in Catholic Apologetics from a university to be an effective Catholic apologist.
Realizing that I had learned Catholicism based upon defective source material was the number one reason for my reversion. When I actually went and read a Catechism I was stunned. When I actually took the time to read some of the documents of the Second Vatican Council II was overpowered by truth and ashamed of my malleable stupidity.
For years, I was taught, and I believed, that the Catholic church's teaching on conscience means we can all follow as much or as little of Catholicism as we, personally, see fit. I was even given isolated quotes from the Catechism that seemed to support this. For example:
1782 Man has the right to act in conscience and in freedom so as personally to make moral decisions. "He must not be forced to act contrary to his conscience. Nor must he be prevented from acting according to his conscience, especially in religious matters."53 [....Snip] Read It All, and save it to your Computer!
14 February: Mary of Veritatis Splendor has made an excellent grammatical clarification to what I had written above: One quibble on language - we do not need to "believe that it is a mortal sin" for it to be a mortal sin; we need to have knowledge of the sin's gravity for it to be mortal. There is a difference there.
Those who do not believe in a "mortal" sin are still quite capable (and perhaps even more likely... Lord only knows) of committing it, in virtue of the natural law that has been given to each of us. Thank you, Mary! That's what I get for declaring that there are no Catholic apologists around here!