The Church teaches that we cant know the true definition of ‘being in a state of grace’. Only God can know the true definition of ‘being in a state of grace’.
No, that’s not true. We know what it
means. We just normally can’t discern whether a particular person at a particular time is actually in a state of grace.
So, if a person is just baptized, or just walks out of confession (having been absolved), we can say “they’re in a state of grace.”
Now, we can talk about ‘grave matter’ as an objective consideration – sins either
are or
aren’t grave matter. That’s pretty easy. However, the next two are
subjective considerations. Therefore, it’s “above our pay grade”, as the saying goes, to determine whether another person has committed a mortal sin. In fact, we’re so good (as humans) at rationalization and other forms of self-deception that it’s difficult for each of us to say definitively
about our own self whether we’re in a state of mortal sin! (St Paul mentions something much like this in one of his epistles!)
So, in summary: we can define all of the things you mention, and there
are times when we can say “yep, he’s in a state of grace!”, but that doesn’t mean that we can proclaim that definitively about a person at any arbitrary time. And moreover, that doesn’t mean that we don’t know the definitions.