Through a glass, darkly

What does Paul mean in his epistle to the Romans VIII chapter, where he speaks of predestination and the Lord's elect? 

He used the terms that could be understood by those to whom he was writing. There are, in fact, no elect as they or he understood it, but truth comes only as the soul of humanity is ready to receive it and he wrote for his time; he understood many things, but as he himself said, he saw through a glass, darkly, and that was one of his most indistinct visions.

—Insight