I have yet to find an example of how “mortal sin” can actually happen. Ignorance and/or blindness are necessary ingredients of all sin, at least that is my viewing. I am very open to an example of how it can happen, but I have yet to find a case of how mortal sin could occur.
Murder, armed robbery, rape,…are some that come to mind. Competent adults can fully be expected to know that these are all serious sins and to know that by committing them they are committing serious sin and yet they decide freely to do so anyway. Our full knowledge and consent does not require more.
(Competent= adult, normal intellectual ability)
Example:
A man decides to rob the corner mini-mart at gunpoint.
He is a competent adult.
-----He is fully aware of the seriousness of this act.
-----He is fully able to give his consent.
He plans it in minute detail.
-----This would indicate that he knows it is seriously wrong.
He especially plans his getaway.
-----He tries to avoid punishment which again means he knows it is wrong.
He pulls the job alone.
-----No one is forcing him, no one is compelling him. He is consenting.
-----He commits the robbery, the serious sin.
(You might ask about his motives, but they don’t matter much in my example. If he was robbing the store to feed his starving children it would still be a mortal sin. Even if you are committing a serious sin “for a good reason” it is totally unacceptable. If he is an addict, he can’t be excused from one serious sin because of another.)
The people of the world are not all philosophers or theologians. They are not expected to be. The commandments are intended to be understandable by all. There ARE times when there is not full knowledge or consent, certainly, but to say that no one can ever commit mortal sin is not true.