While a lector at Hagia Sophia, Theodorus collected the works of the fifth-century historians Socrates Scholasticus, Sozomen, and Theodoret of Cyrrhus to create a chronicle of church history from Constantine to Theodosius II. The resulting work, Selections from Church History (), known better by its Latin title Historia Tripartita, is a single narrative in four books which gives Theodorus' preferred reading for each section of history related, with notes and comparisons in the margins.
