A much-quoted phrase from the Bible, usually considered to mean that, as humans, our vision of events is often obscured, as if we were observing them in a darkened mirror. When Lestat tells Louis: "You no longer look 'through a glass darkly,'" he is telling Louis that he cannot return to the mortal world with his newly acquired knowledge: Louis can no longer experience mortal life. His glass is no longer dark.
When I was a child, I spake as a child, I understood as a child, I thought as a child: but when I became a man, I put away childish things. For now we see through a glass, darkly; but then face to face: now I know in part; but then shall I know even as also I am known. And now abideth faith, hope, charity, these three; but the greatest of these is charity.
1 Corinthians 13:11-13