Greek theologian, writer, and prominent head of Alexandria's Catechetical school, at that time the intellectual centre for Hellenistic Christianity. Reputed to be one of the Greek Church's distinguished teachers, Theognostus assumed the leadership of the school c. 265, although the precise line of succession is not certain. His principal work, the Hypotypōseis (Greek: “Outlines”), is a doctrinal compendium in seven books intended for use at the school.

Adhering to the teaching of Origen, a theologian of the 2nd–3rd century, Theognostus organized his work and adopted his terminology from his master's Peri archōn (“On First Principles”). The Hypotypōseis was acclaimed by Gregory of Nyssa, a 4th-century intellectual leader of the Eastern Church, but was sharply attacked almost five centuries later by the Byzantine patriarch Photius of Constantinople, whose Myriobiblion (“Library”), or Bibliotheca, has preserved the fullest account of the work. Interpreting Theognostus' text as subordinating the Son and the Holy Spirit to the Father, Photius scorned what he considered Origenistic views of the divine Trinity. Yet the orthodox Athanasius of Alexandria appealed to the Hypotypōseis during the 4th-century controversy with Arianism, a heretical movement teaching that Christ is inferior to the divine nature, being the human form of the created Logos (Word).

The Hypotypōseis reflected other Origenistic opinions contrary to Neoplatonism, such as the noneternity of matter and the rational possibility of Christ's incarnation, tenets characteristic of the Christianized Hellenistic philosophy at the Alexandrian school. Despite his criticism, Photius still commended Theognostus' treatment of Christ's redemptive work and admired the clarity of his Attic literary style. An English translation of the extant fragments of the Hypotypōseis is contained in the collection The Ante-Nicene Fathers, Alexander Roberts (ed.), vol. 6 ( 1885). Newly discovered remains of the second book were published by Franz Diekamp in 1902.