For, agreeably to what we have already explained as to the manner in which He is the Way, and conducts to the Father; and in which He is the Word, interpreting the secrets of wisdom, and the mysteries of knowledge, making them known to the rational creation; and is also the Truth, and the Life, and the Resurrection,— in the same way ought we to understand also the meaning of His being the brightness: for it is by its splendour that we understand and feel what light itself is. And this splendour, presenting itself gently and softly to the frail and weak eyes of mortals, and gradually training, as it were, and accustoming them to bear the brightness of the light, when it has put away from them every hindrance and obstruction to vision, according to the Lord's own precept, “Cast forth the beam out of your eye,” renders them capable of enduring the splendour of the light, being made in this respect also a sort of mediator between men and the light. (Book I, Chapter 2, Part 7)

Brightness allows us to accurately perceive.

Christ is the aspect of God which empowers our ability to see.

Without Christ we might hear, touch, taste, or smell but reality would be enshrouded.

Even with Christ we may only see as through a mirror, darkly.

But as our experience of Christ grows - as the brightness increases - we will see as if fully face-to-face.