Panel 1: The Old Testament cycle of the nave begins at the point where the southern wall meets the wall of the apse. The scene depicts "The offering of bread and wine to Abram by Melchizedek" (Gen. 14:18-20). It occupies the entire panel. This is unusual in the cycle, in which we usually find two scenes sharing a single panel, one over the other. The presence on high of a bust of Christ, emerging from the clouds and pointing to the offering of Melchizedek, helps us to grasp the typological value attributed to the episode, which is an anticipation of the Eucharist. Typologically, the scene recalls the Christological cycle of the adjacent Triumphal Arch. This begins a series of panels showing the life of Abram.
(From: Francesco Gandolfo, "La basilica sistina: i mosaici della navata e dell'arco trionfale," in Chiese Monumentali d'Italia. Santa Maria Maggiore a Roma, edited by Carlo Pietrangeli [Florence 1988] p. 88 [translated by Bernard Frischer]).