From the Explanations of the Psalms by St Ambrose, bishop

THE LIGHT OF YOUR FACE HAS SHONE UPON US

Why do you turn your face away? We think that God has turned his face away from us when we find ourselves suffering, so that shadows overwhelm our feelings and stop our eyes from seeing the brilliance of the truth. All the same, if God touches our intellect and chooses to become present to our minds then we will be certain that nothing can lead us into darkness.
A man’s face shines out more than the rest of his body and it is by the face that we perceive strangers and recognise our friends. How much more, then, is the face of God able to bring illumination to whoever he looks at!
The apostle Paul has something important to say about this, as about so many other things. He is a true interpreter of Christ for us, bringing him to our understanding through well-chosen words and images. He says: It is the same God that said, ‘Let there be light shining out of darkness’, who has shone in our minds to radiate the light of the knowledge of God’s glory, the glory on the face of Christ. We have heard where Christ shines in us: he is the eternal brilliant illumination of souls, whom the Father sent into the world so that his face should shine on us and permit us to contemplate eternal and heavenly truths – we who had been plunged in earthly darkness.
What shall I say about Christ, when even the apostle Peter said to the man who had been lame from birth Look upon us? The cripple looked at Peter and found light by the grace of faith: unless he had faithfully believed he could not have received healing.
When there was so much glory to be seen among the Apostles, Zachaeus, hearing that the Lord Jesus was passing by, climbed a tree because he was small and weak and could not see the Lord through the crowd. He saw Christ and he found light. He saw Christ and instead of robbing others of their goods he began to give away his own.
Why do you turn your face away? Let us read it thus: even if you do turn your face away from us, Lord, its light is still imprinted upon us. We hold it in our hearts and our innermost feelings are transformed by its light.
For if you truly turn your face away, Lord, no-one can survive.