In the foreground, Christ celebrates the Passover liturgy inside an octagonal chamber. It just so happens that in Rosselli’s day an edifice with this same shape was unearthed on the Aventine hill—none other than the Domus Aurea, Nero’s imperial villa. By cladding the Upper Room in this way, Rosselli encoded a message in his fresco: the eternal high priest, Jesus Christ—ever ancient, ever new—reigns today as the King of kings.

   But look to the windows for the real stroke of genius. The same Christ who blesses the cup simultaneously prays in the garden of Gethsemane (left window), is arrested (central window), and is crucified (right window). The three chronologically ordered frames form one single landscape, linked by a winding footpath.

What is Rosselli trying to say? 

Half a millenium later, Vatican II seemed to answer that question: "As often as the sacrifice of the Cross by which ‘Christ our Pasch has been sacrificed’ is celebrated on the altar, the work of our redemption is carried out." (Lumengentium3).

   The Bible comes to life in the liturgy, which re-presents and re-actualizes the mysteries it celebrates. It’s an idea that goes back to the Old Testament. The Jews celebrated God’s interventions with a memorial that made present what it signified. Here and now. To me! The Passover, for instance, was commemorated down through the centuries by saying, “It is because of that which the Lord did for me when I came forth out of Egypt” (Ex 13:8). 

   Christians inherit this patrimony. “In the liturgy of the Church, it is principally his own Paschal mystery that Christ signifies and makes present” (CCC 1085). 

   So if at Mass you would wonder about what is going on beyond those stained-glass windows, know that your musings pale in comparison to the true splendor. See God’s saving events flash before you. The Lord thrusts the planets into their orbits, breathes Adam into life, tosses Noah up and over the billowing waves, tears them apart to let Israel pass through the Red Sea, rains down manna from heaven. The snapshots come faster now. A star, a manger, a babe. Palms and a donkey, Judas and Malchus, dice and a tunic. Mourners in a rock-hewn tomb, angels in a garden, apostles in the upper room. “Peace be with you!” Christ ascends on a cloud; The Holy Spirit descends in fire. 

   Aware of an ineffable majesty, the priest rises after the words of consecration and can barely utter, “The mystery of faith!” 

   This sacramental vision is as dizzying as it is glorious. How can I be there? Isn’t the Paschal mystery a real event that occurred “once and for all” in history? 

   Yes, “but it is unique: all other historical events happen once, and then they pass away, swallowed up in the past. The Paschal mystery of Christ, by contrast, cannot remain only in the past, because by his death he destroyed death, and all that Christ is—all that he did and suffered for all men—participates in the divine eternity, and so transcends all times while being made present in them all” (CCC 1085).

   So if you want to wonder what the angels behold at Mass, look out the windows. Be distracted! Just don’t tell your parish priest I said so.