(I once was what you are and what I am you also will be). This memento mori underlines that the painting was intended to serve as a lesson to the viewers. At the simplest level the imagery must have suggested to the 15th-century faithful that, since they all would die, only their faith in the Trinity and Christ’s sacrifice would allow them to overcome their transitory existences.
According to American art historian Mary McCarthy:
The fresco, with its terrible logic, is like a proof in philosophy or mathematics, God the Father, with His unrelenting eyes, being the axiom from which everything else irrevocably flows.
Source: McCarthy, Mary (August 22, 1959). “A City of Stone”. The New Yorker. New York: 48.