Through a glass darkly ... I suppose I'd always assumed that the reference was to those smoked glass sheets through which people are advised to view eclipses; to, in other words, the necessary and protective occlusion of the childish imagination. To prevent (say) our wits being blinded or blasted by the unaccommodated nuclear glare of Reality. The dazzle that produces dervishes and fanatics.
Of course it's mirror; and though there's something nicely uncanny in the idea that the Reality we observe is in fact through a mirror, yet I feel a little sorry to have to let go my childish darkened understanding of what the phrase means.