Why and in what ways is it permitted to humans to own anything at all, privately or otherwise? This dissertation draws out and systematizes general principles from across the Christian tradition to answer this fundamental question. It offers, in other words, a Christian theory of property: identifying key principles from patristic discourse; how these principles are explicated and systematized in twelfth- and thirteenth-century canon lawyers and theologians; and why this tradition offers a compelling lens to interpret Catholic Social Teaching.For the Christian tradition, property is a political and only secondarily an economic institution, aimed at the achievement and preservation of political and moral ends in human social life and only secondarily at economic values like productivity, efficiency, and development. The foundational principles are consistent across Basil, Ambrose, Augustine, and Chrysostom: the Earth and all things belong to God (Psalm 24:1); God has given all things for the sustenance of each person; and property is a power of distribution and not personal use. Grounded in these principles, the scholastics hold that property is something invented by humans to preserve God's intentions for the world. Property is, in other words, a convention, aimed at preserving the principles of natural equity and equality and the sufficiency and liberty of each person—what I call in this dissertation the natural law of common dominion. Catholic Social Teaching's (CST) position on property suffers from a lack of clarity and clear connection to this long Christian tradition. Since Leo XIII's use of a Lockean labor-mixing theory of property in Rerum novarum, there is a tendency to read CST on property as Lockean. In the penultimate chapter I show why Locke offers an unsuitable lens for interpreting CST: his approach is logically incoherent, conducive to imperialist thought and practice, and, contrary to the patristic and scholastic traditions, holds that property is primarily a power of personal use. In the final chapter, I offer an interpretation of CST from Rerum novarum to Fratelli tutti, showing how the theory of property I recover from early church and medieval sources can make sense of this diverse body of literature and many of its diverse commitments, from the social character of property to its commitment to the self-determination of peoples.