The "fact of revelation" so to speak, is that man as he is, is a fallen creature. He is born into bondage, left to his own devices, he will not fail to displease God. It's not self-help, positive-affirmation stuff, but I think most honest people will realize that people as they are (and if they're really honest, themselves in particular) are pretty crummy. Even the best of us, are capable of the most horrid things - and the tendency to be this way is pretty much "died in the wool", a potential that is there from the cradle.
Also, it could be pointed out that "the flesh" in and of itself, cannot inherit "the spirit" (as those terms are understood in Scripture) - with that said, the end of "heaven" cannot be inherited by that which is not in the grace of God.
It really goes back to that old, scandalous teaching of the Fathers "no salvation outside of the Church", which at it's heart is rooted in the notion of there being no salvation apart from the Lord Jesus Christ. He is the "new humanity", and if one is not a participant in this, they're members of something fallen, and destined for a bad end since at best, it has nothing going for it in eternity (and at worst, has all manner of willful sins to wound the conscience with.)
Now the problem is, those things are all easy to say, easy to assert, but difficult to justly apply. Various people (including Church Fathers) have made attempts to concretely apply them (such as both Sts.Augustine of Hippo and Cyprian of Carthage), struggling along the way with the obvious "hard cases."
Which is precisely why, at the end of the day, while you see Christians historically saying that there are some amongst them who they undeniably believe (based on a pious life and miracles) have inherited their reward in Heaven, they are not nearly this sure when saying who is or isn't in hell (though most would seem to say it's a safe bet that Judas Iscariot is - after all, what else could merit it being said by the Lord that it would have been better he, Judas, had been not born at all?).
Thus while it doesn't satisfy curiosity, I think the only actual answer to those hard cases is to say little if anything at all. It just seems to me that when folks don't do this, they go to one extreme or another - they either diminish the holiness of God and the horror of sin and the shadow it casts over things, or at the other end God is portrayed in a way that makes Him seem unaware of even the most basic standards of justice.