If there is an iconographic image to illustrate what St. Gregory the Theologian is trying to explain in Section 4 of his Oration 38, it is this close-up of the Icon of the Nativity:
Please note how Christ is depicted in the manger. He is not lying in a feeding trough lined with hay nor is he wrapped in swaddling clothes. Rather, He is lying in the dark, beneath the earth and in a tomb swathed in burial wrappings. This is what we celebrate on Christmas — Christ crucified and buried.
The entire purpose of Christ’s Incarnation is His suffering and death so as to intimately identify Himself with our suffering and death. That way we can pick up our cross, die with Him and allow His humanity — which knows no corruption — can become our humanity. So that, as St. Gregory puts it, we may lay “aside the old human being [so that] we may be clothed with the new, and that as in Adam we have died so we may live in Christ, born with Christ and crucified with him, buried with him and rising with him.”
The Orthodox Church depicts the Nativity with Christ in the tomb to visually remind the world that “where sin abounded, grace superabounded.” Death has no sting. We don’t celebrate those things that we want or that we think we need (what St. Gregory calls sickness), but rather the healing that the Cross and Crucifixion bring. We celebrate the astonishing love of God.