Seen in person at Santa Maria Novella in Florence.
The classical elements are not superfluous. The hallway supported by arches suggests the Father is walking towards us, except He is holding the cross bearing His Son. The dove, the Holy Spirit, is between Father and Son, but approaches vertically as opposed to horizontally. Our eyes are drawn in both an opposite horizontal and vertical direction. Horizontal motion invites us to place ourselves with the patrons kneeling, or attempt to step beyond them.
The Holy Spirit descends, but we ascend: we see the family crypt of the patrons, then the patrons themselves. They are on the outside looking in, separated – made almost invisible – by the pillars. Mary is visible to all, inviting us to observe the crucified Christ. What motivates John is invisible: he is lost in contemplation. The vertical movement leads to a contrast between divine and human death. We know Christ will rise; we know we must die. We relate to God through God dying: God cannot be stepped up to. The patrons are as far as we can get.