Claim. Because Adam failed to reach the tree of life, perfected humanity must come to earth in his place. Jesus came as this tree of life — the “last Adam” — and humanity is engrafted to him. But the engraftment was only spiritual; original sin still passes through lineage, so Christ must come a second time on earth as the tree of life of Revelation to complete the redemption physically.
Elaboration. Per 1.1.1. The Tree of Life: “Burdened by the original sin, fallen people cannot complete the ideal of creation and become trees of life by their own efforts alone.” A perfected man must come, and “all of humanity must then be engrafted with him.” Jesus was that man. “The tree of life for which the faithful people of the Old Testament Age longed was none other than Jesus.”
But spiritual engraftment alone leaves original-sin in the lineage: “the children of even the most devout Christian parents still inherit sin.” Therefore Christ must return on earth as the tree of life of Revelation, “engrafting all humankind with himself once more” to redeem original sin at its root. The first tree of life (Eden, unrealized) and the last tree of life (Revelation, to-be-realized) bracket the entire providence of salvation.
This is DP’s distinctive christology: Jesus’s mission was the tree of life mission; that mission required a physical (not just spiritual) completion which only the second-advent can deliver.
See also. dp-original-sin-transmitted-via-blood-lineage, dp-pentecost-as-flaming-sword-removal-by-holy-spirit