What constitutes "grave matter"?

[Sy_Noe]

I have a bit of a twist on this. What does it mean to have full knowledge and deliberate consent of the grave matter? More specifically, if a person is aware and fully knows the Church considers an act to be a grave matter and knows the Church’s reasons why they believe such a thing to be a grave matter, and yet they deliberately consent to the act because they do not believe in what the Church teaches in regard to a particular act being grave… Is that person in mortal sin? Or are they not because they don’t believe it so they don’t fully know. And then can such a person receive Jesus at the Eucharistic table as long as they discern His body and blood of course.