Let’s begin with the three requirements for an action to be mortally or deadly sinful (1 Jn 5:16). First of all it must objectively consist of grave matter. It must be objectively mortally sinful such as murder or sacrilege. Secondly, one must have knowledge of this fact. Thirdly one must freely will it. If any one of these requirements is missing, then the action is not mortally sinful for the individual.
So if you take an action that is objectively a mortal sin and do not know that it is a mortal sin, you don’t have the necessary knowledge and you are not guilty of a mortal sin. However, some actions we automatically know are gravely wrong because of what is called the Natural Law. For example, we automatically know that stealing is wrong or that killing is wrong.
It is possible that our consciences are dull or uniformed in some areas. We are obliged to inform our consciences as best we can. For example as a Catholic, one cannot deliberately keep oneself in ignorance about the Church’s moral teaching so that one is not bound by it. This, in itself, would be a grave sin.
You mention believing that fornication is not a mortal sin. While this can be true if one is so steeped in the pagan culture in which we live and has known nothing else, he possibly may not be morally culpable. But this would be a difficult one for a Catholic who has been informed of the Church’s teaching on the matter. To put oneself above the teaching of the Church, whose authority one believes comes from Jesus Christ Himself, can be an arrogant act and a sin of pride. It is also an act against one’s faith.
For some people whose sense of morality is so numbed by such habitual sinful behavior, they may not be culpable for each subsequent sinful act because the morality of their actions is no longer a matter of consideration. But such a state is nevertheless a morally deadly one.
On the other hand, if one mistakenly thinks an action is objectively sinful when it isn’t and wills it nevertheless, then it IS sinful for Him because moral actions come from the will. St. Thomas Aquinas says that if you want to be a saint, will it!
Fr. Vincent Serpa, O.P.