For now we see in a mirror, dimly, but then face to face. Now I know in part, but then I shall know just as I also am known.
During this physical life, Christians have only a glimpse into the spiritual realm.
William Barclay gives this background about the mirrors of the first century: “As things are, what we see are reflections in a mirror. That would be even more suggestive to the Corinthians than it is to us. Corinth was famous for its manufacture of mirrors. But the modern mirror as we know it, with its perfect reflection, did not emerge until the thirteenth century. The Corinthian mirror was made of highly polished metal and, even at its best, gave but an imperfect reflection” (William Barclay, The Daily Study Bible Series, note on 1 Corinthians 13:8-13).
But we have the promise of a future time when we will see and understand and know God fully. Christians receive the Holy Spirit now as a down payment of that future change to spirit life at the first resurrection.
For more on this, see the article on “Resurrections: What Are They?”