Byzantine monk and theologian who provided a breakthrough of terminology in the 6th-century Christological controversy over the mode of union of Christ's human nature with his divinity. He did so through his introduction of Aristotelian logical categories and Neoplatonic psychology into Christian speculative theology. His work initiated the later intellectual development of Christian theology throughout medieval culture.

Leontius became a monk while young and took active part at Rome in the theological disputes of the time. Moving to a new monastery near Jerusalem c. 520, he returned to Constantinople in 531 to participate in a conciliar meeting on the Christological question and, c. 542, to seek judgment in a dispute over monastic theology.

In the controversy over Christ, Leontius at first tended to favour the Nestorians. Exposing certain of the monophysites' (q.v.) fraudulent uses of patristic authorities, Leontius criticized them and the followers of Eutyches (q.v.). Later, however, in his principal work, Libri tres contra Nestorianos et Eutychianos (“Three Books Against the Nestorians and the Eutychians”), he assumed a moderate, orthodox position, having been influenced by the leading Nestorian adversary, Cyril of Alexandria.

The “Three Books,” a primary source for verbatim expressions of the various theological schools, develops the concept that eventually played the key role in reaching a mediatory orthodox formulation at the general council of Constantinople in 553, thus integrating the partial conclusions from the previous councils at Ephesus in 431 and at Chalcedon in 451.

Involved with promoting the monastic influence of Origen (q.v.), Leontius was the object of a negative judgment at Constantinople.