In the mid-sixth century, Cassiodorus wrote his Institutiones Divinarum et Saecularium Litterarum to instruct the monks at Vivarium in their scribal work of collecting, codifying, and copying the Christian Scriptures, along with a vast array of Latin Christian literature. His text remained an essential handbook for monks and nuns working as scribes for centuries. Within it, he includes three authoritative canon lists which he takes from Jerome, Augustine, and the Septuagint. To modern scholars these lists often read as nonsense: he seems entirely ambivalent towards which books are “in” or “out” of the canon, he appears unfaithful to his source material, and none of these lists reflects his own system for listing or grouping the Scriptures. What then is the point of them? The answer lies in the importance that Cassiodorus, and other late antique authors, place on numbers as sources of allegorical interpretation in the search for higher meaning. Through a process of “holy arithmetic”, Cassiodorus presents what he claims is an inner logic of these authoritative canon lists, bringing to light three different hermeneutical lenses for understanding what the Scriptures are. As allegories, those lenses can coexist in a complementary fashion, aiding Cassiodorus in his larger mission to codify a Latin Christian tradition. Examining Cassiodorus’s approach to listing the canon and comparing it to modern scholarship on the subject bring into focus some of the key ways in which our own assumptions and methods differ from those of our late antique sources. It also opens up new possibilities for interrogating these sources.