In his treatise de Trinitate or The Trinity, St. Augustine referred to “the back parts” of God. Here he is referring to the flesh of God which was born of the Virgin Mary, suffered, died, and rose again. The face of God cannot be seen this side of heaven. “Not therefore without cause will no one be able to see the face, that is, the manifestation itself of the wisdom of God, and live” (de Trinitate Book II, C. 17). Therefore, it is fitting for St. Augustine to refer to what we can see as God to be “the back parts.”
“The back parts” can be seen in the Catholic Church as His presence in the Holy Eucharist which is His real flesh.
“The back parts” of God can be seen in all humanity for those who have eyes that see divine things. Thus, the suffering face of Christ can be seen wherever the poor and marginalized are found. But the face of God can only be contemplated, not seen.
The presence of God is rarely recognizable until after the resurrection. Like St. Peter, many people can walk with Christ but few have eyes to see divine things. It is post-resurrection that God removes His hand and allows “the back parts” to be seen.
For as to that, too, which follows in Exodus, I will cover you with mine hand while I pass by, and I will take away my hand and you shall see my back parts; many Israelites, of whom Moses was then a figure, believed in the Lord after His resurrection, as if His hand had been taken off from their eyes, and they now saw His back parts. (de Trinitate Book II, C. 17)
The Old Testament hinted at the mystery of the Holy Trinity, but not until the coming of Christ was the presence of God recognizable as three persons united in one Godhead. In order to awaken to “the back parts” of God, human creatures must come to recognize how profoundly God’s grace has acted upon their lives. That is why no one can see the face of God and live.