We have seen how Azrael’s dagger made ordinary humans into murderers with a bit of amnesia. We then learnt the dagger used to be the Flaming Sword that guarded the garden of Eden and that God destroyed it after the rebellion.
When God Johnson picked up another piece of the sword, something in him changed. He had memories he shouldn’t have of God’s celestial family and healing abilities. Amenadiel thinks the healing powers lead Johnson to assume he was indeed God, but what it the Medallion of Life in fact not only contained the power to heal but also part of Dad’s memories? It would explain how all memories Johnson had were of happier times, before Lucifer’s fall. It also explains how God’s memories and powers were all gone when the belt-buckle was taken from him.