The collection of Theology begins with nearly a thousand Bibles, mostly Greek; a fine series of the liturgical books of the Orthodox Church, together with samples of the other Eastern rites and the Latin Church. The Church Councils, Canons, and Catechisms complement numerous editions of the Greek Fathers of the Church (first to fifteenth century) and later theologians; the history of the Churches, schisms and religious polemics, and, more recently, efforts towards reunion. Judaism, Islam and other faiths round out the riches of this collection.

The origin of the printed Bible begins for us with the Greek and Latin Psalter printed in Milan, 1481; the Aldine Greek Psalter ca. 1496; the New Testament in Greek and Latin, edited by Erasmus and printed by Frobin in Basel, 1516; and the complete Bible in Greek, produced by the Aldine press in 1518. The Library also owns the translation of the New Testament in Modern Greek, printed in Geneva in 1636, and sponsored by Kyrillos Loukaris, patriarch of Constantinople, who was strangled later that year.

Today the collection is being enriched with books on the history of Christianity, the Greek Orthodox Church, Bible annotations, etc.