It's located in St. Apollinare in Classe, Italy. Classe is near Ravenna, which has some spectacular mosaics, including these two which I already use in Catechism class to discuss Abel, Abraham and Melchizedek as they show up in Genesis:
Two scenes from Abraham's life
Abel and Melichizedek
They're both in San Vitale, which was consecrated in A.D. 546. St. Apollinare was consecrated in A.D. 549, so it's no surprise that the art is similar.
Anyway, per this older post I use these mosaics in September to lay the groundwork for discussing this line from the Mass in April:
"Be pleased to look upon these offerings with a serene and kindly countenance, and to accept them, as once you were pleased to accept the gifts of your servant Abel the just, the sacrifice of Abraham, our father in faith, and the offering of your high priest Melchizedek, a holy sacrifice, a spotless victim."
But next year I'll use the St. Apollinare mosaic to discuss the Mass instead of using the two San Vitale mosaics twice in the same year. I'll point out that the mosaic of all 3 men and their offerings together around a common altar directly illustrates that line in the Mass. It shows that early on the liturgy was explicitly connecting their sacrifices to the sacrifice re-presented at Mass. Like a lot of things in Catechism class, I can always tell this to the kids. But when they can see it for themselves they learn better.
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By the way I checked online for the oldest extant text of a Mass, and found the Gelasian Sacramentary from about A.D. 750. It says:
"...et accepta habere, sicuti accepta habere dignatus es munera pueri tui iusti Abel, et sacrificium patriarchae nostri Abrahae, et quod tibi obtulit summus sacerdos tuus Melchisedech, sanctum sacrificium, immaculatam hostiam."
In English:
"...and accept, just as
you deigned to accept the offering of your just servant
Abel, and the sacrifice of our patriarch Abraham, and that which your highest
priest Melchisedech offered you, a holy sacrifice, a unstained
offering." (thanks to Steve Perkins)
