From the following article:
But they also had the biblical narrative of the "pagan" Cornelius who, the Acts tell us, was "an upright and God-fearing man" even before baptism. Gradually, therefore, as it became clear that there were "God-fearing" people outside the Christian fold, and that some were deprived of their Catholic heritage without fault on their part, the parallel Tradition arose of considering such people open to salvation, although they were not professed Catholics or even necessarily baptized. Ambrose and Augustine paved the way for making these distinctions. By the twelfth century, it was widely assumed that a person can be saved if some "invincible obstacle stands in the way" of his baptism and entrance into the Church.
Considerations in that discussion would include asking to what extent God imposes Himself on the free will of all concerned at their deaths, "enlightening" them outside of their life-long experience to bring them to some form of saving faith; how, if this is done with great frequency by God, this does or does not circumvent the real "necessity" of receiving sanctifying grace through the Sacraments; how, indeed, this sort of deathbed enlightenment of everyone doesn't make a mockery of the Cross, and how it can square with the universal call to holiness and the careful walk described in Matthew 7:13-14.
I am not an exclusivist, I assure you. But I think it is rash for us to simply assume the wide availability of deathbed enlightenment and conversion for those deprived, for one reason or another, of sacramental grace. Again, the presence of sanctifying grace is essential to gain Heaven, and we are told clearly by Our Lord Himself how to attain that grace. That He may go outside of His own established norms is truly a given - He is God. But it is prudent to suppose that He meant what He said, and that what His Church, ordained by Him as the pillar and bulwark of the truth, has taught throughout the Christian Era in regard to the Sacraments is wholly in harmony with His will.
The bottom line is that we cannot know with certainty to what extent, if any, God goes outside of His own established norms. He has not chosen to reveal that to us, and it is thus outside of the Deposit of Faith. So it is prudent to assume that even the non-sacramental Christians will have a much more difficult time dying in a state of perfect contrition - and thus having sanctifying grace in their souls - than any of us would find it comfortable to suppose.
Are there non-Catholic Christians in Heaven? Almost certainly. Are there non-Christians in Heaven? Quite possibly. But, how many? Only God knows, and He ain't tellin'! The Church, in the meantime, would do well to suppose that "ordinary means of salvation" means just that, and to continue evangelizing accordingly!