Full knowledge does not mean that the person explicitly recognizes an act as mortal sin, but that he is or should be aware that the act is seriously wrong. Choosing to be ignorant about the moral character of an act in no way diminishes the person’s responsibility for the act.
This ignorance can often be imputed to personal responsibility. This
is the case when a man ‘takes little trouble to find out what is true
and good, or when conscience is by degrees almost blinded through the
habit of committing sin’ (GS, 16). In such cases, the person is
culpable for the evil he commits (CCC, no. 1791).
Man not only has a conscience to guide him, but a responsibility to properly inform that conscience. While “ignorance of Christ and His Gospel, bad example given by others, enslavement to one’s passions, assertion of a mistaken notion of autonomy of conscience, rejection of the Church’s authority and her teaching, lack of conversion and of charity” can lead to erroneous judgments of conscience, they do not necessarily constitute invincible ignorance (CCC, no. 1792). Invincible ignorance means that the person is not responsible for their lack of knowledge. It is only this type of ignorance that makes the person not responsible for his morally evil act. However, even in this case, the act “remains no less an evil, a privation, a disorder” ( CCC, no. 1793).
The meaning of man being made in the image of God does not end, however, with reason and will. Man is, by his very nature, made for relationship with God. The missionary mandate is to bring man into full relationship with God. Therefore, to neglect Christ’s mandate is to neglect our fellow man.