I know a good number of people who didn't attend mass regularly or who look at pornography, but are not aware that they commit a sin. Actually, it might be pretty hard for the average person to commit grave sin, for who would purposely turn against God?
For a sin to be mortal, three conditions must together be met: grave matter, full knowledge, and deliberate consent (cf Catechism 1857). However, there are aspects of this question that bespeak troubling trends in modern thinking.
First, there is the notion that people don't seem to know any better. Scripture, Sacred Tradition, and the Catechism all speak of the conscience in every human person. The voice of God echoes in the depths of every human heart. While some suppress this voice, deep down it is still there. There is much pastoral experience that people generally do know what they are doing. When speaking to people who are missing Mass, or perhaps are cohabiting and fornicating, etc., they admit they know, deep down, what they are doing is wrong.
Secondly, the notion that mortal sin is rare also seems rooted in modern anthropology that minimizes human freedom and knowledge. While it is true, certain compulsions may marginalize or limit freedom, yet, we are freer than most like to admit. In summoning us to a moral life, and warning us of sin, the Lord in Scripture is not simply setting up a straw man. He is speaking to us as moral agents, who generally act freely, making decisions for which we are responsible.
One may call all this “needless fear,” but if so, the Lord never got the memo. Jesus often used vivid imagery to stir fear within us of the consequences of sin. As with any pastoral appeal, fear must be balanced with other appeals as well. But the modern attempt to remove all fear from the preaching of the Church, has had poor results. Some degree of fear may be “needful” after all.
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