Does Lack of Belief Negate "full knowledge" for mortal sin?

CCC 1857 says: For a sin to be mortal, three conditions must together be met: “Mortal sin is sin whose object is grave matter and which is also committed with full knowledge and deliberate consent.”

What constitutes “full knowledge”?


  1. Assume that I am a baptized Catholic.
  2. Assume that I know that the Church teaches that fornication is gravely sinful.
  3. Assume further that I simply do not believe that the Church’s teaching is true.
  4. Assume also my lack of belief is sincere, even if misguided.
  5. Finally, assume that, following my conscience, I with deliberate consent engage in acts that I know the Church says are gravely sinful.

Does my lack of belief (or perhaps a failure to appreciate the reasons for the prohibition against fornication) equate to lack of “full knowledge” so that I remain in a state of grace and have not committed a mortal sin?

Whether or not the individual will be culpable of the grave sin will depend on the reason why he is making the erroneous judgment. It could be he is responsible for his erroneous judgment. For example, he may be allowing his desire to continue fornicating cloud his judgment. In this case, his ignorance would not be invincible. He may have never given any serious thought to why this act may be a violation of the goods of human sexuality. Furthermore, his erroneous judgment could be due to the fact he has not taken the time to consider the rational grounds for the authority of the Church, which justifies her teaching. If that were the case, his erroneous judgment would not be excusable.

It is true we must always act in accord with what we judge in the intellect to be good. But it is also true we are obliged to form our intellects in order that we can make appropriate judgments. Being that this individual is a Catholic, he would be bound to not only form his intellect with truths he can know by the natural light of human reason, but also the teachings of the Church.

Only God knows the reasons behind the individual’s erroneous judgment. If those reasons are not sufficient for invincible ignorance, then he would be culpable for the sin.

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