In the sanctuary of St. Martin's, behind the old High Altar, is a Eucharistic "trinity" of windows. These are perhaps the most beautiful, and definitely the most profound, windows in the entire church. Each features a victim or offering being sacrificed on an altar, by a person acting as priest. The priest-king Melchisedek offers bread and wine on an altar of stone. With Isaac as victim, Abraham acts as priest in sacrificing his only son on a stone altar topped with wood. Christ is both Priest and Victim on the altar of the Cross at Calvary (from calvaria - "skull", the Latin translation of the Aramaic Golgotha). The eyes of Abraham, in the left-hand window, and the eyes of Melchisedek, on the right, look directly at the Crucifixion in the center window. The events depicted in the two side windows foreshadow the event of the center window, and thereby also foreshadow what occurs each time the Holy Sacrifice of the Mass is offered on the Altar below. 

The side windows also remind us of other sacrifices in the Old Testament. The Israelites sacrificed and ate a paschal lamb during the Passover memorial, and brought offerings to the Temple on other days. Each year on the Day of Atonement, the high priest sacrificed a goat and entered the Holy of Holies, sprinkling its blood to atone for the sins of his people {see Leviticus 16:15-19}. 

But the many sacrifices by the priests and people of the Old Testament were only anticipations of the one perfect Sacrifice of Jesus on the Cross. On the night before that Sacrifice, as we saw in the Last Supper window, Jesus gave Himself to the Apostles as the new Paschal Meal. As our High Priest, He "entered once for all into the sanctuary, passing through the greater and more perfect tabernacle not made by hands, that is, not belonging to this creation. He entered, not with the blood of goats... but with His own blood, and achieved eternal redemption" {Hebrews 9:11-12}. 

- St. Thomas Aquinas (1225-1274)