To some it is given by the Holy Ghost to know that Jesus Christ is the Son of God, and that he was crucified for the sins of the world.
To others it is given to believe on their words, that they also might have eternal life if they continue faithful.
If something as basic as a knowledge that Jesus is the Son of God is not available universally, then I think there isn't much that is available universally - and that we should be totally accepting of those to whom it is given to believe (to whatever small or large degree they are given to believe). I also think we need to be just as accepting of those who only can desire to believe - who live as faithfully as they can in relative darkness.
I believe in the grand scheme of things, those who can't say they know but continue to believe or strive to believe have lived every bit as "valiantly" (if not more so) than those who have been given to know and live according to that knowledge.