In a previous post, I argued that (apart from canonizations and private revelations) we cannot be certain, either by faith or by knowledge, of anyone's eternal fate. Let me mention a few consequences of this fact that may suggest why it is fitting that we lack the capacity for such certainty.
The first reason is related to Brandon Field's comment on my earlier post:
I'm pretty sure that people who want to rejoice at the idea of another person in Hell don't have an accurate concept of Hell.
If some combination of faith and reason provided the means for us to be certain that a particular person is in hell, then sooner or later the doctrine of damnation would become dull. As it is, we've done a good job of losing interest in the Four Last Things. I'm not sure how much of that is due to unfounded certitude, but I think it would be even worse if we were capable of certainty.
Just look at how dull we find the dogma of the Resurrection of Jesus from the Dead. We can barely make it through an Octave before the Resurrection is buried for another year. That, at least, is a dogma we need to know; certainty in the fate of any one person is not.
Another consequence of uncertainty is that we must respect the virtue of hope. Hope lies in the mean between despair and presumption. It's not easy to maintain that balance. Being uncertain about the fate of anyone else helps train us to hope in (rather than presume or despair of) our own salvation.
A third consequence is that we must respect the freedom of others throughout their lives. Though sin weakens the will and vice makes it harder to choose the good, still we believe that up until the moment of death every human being is capable of turning to God, however imperfectly, in repentance and love. This capacity is something we must acknowledge, in others as well as ourselves.
Another consequence is that we ought to be humbled by our limitations. We don't, we can't, know everything about the life to come. We can't pass a final judgment on anyone. We are not God; we are not His deputies; we are not His advisers; we are not His confidants. We are His servants and His children, and that only by His wholly gratuitous love for us.
The final consequence I'll mention is that we cannot mark the limits of God's mercy. We must confess that it is possible that the worst sinner, with no outward sign, be given the grace to ask for and obtain God's forgiveness. This should be cause for wonder and awe -- as well as the humble acknowledgement that our own hope for salvation lies wholly in God's mercy and not at all in our own virtue.