St. Gregory of Sinai lived during the late 13th to early 14th Century. He lived on Mount Athos as a monk for 25 years in a secluded hermitage. And he wrote numerous treatises on Christian theology and spiritual practice.
Here's St. Gregory's succinct definition of Christian Orthodoxy:
"Orthodoxy may be defined as the clear perception and grasp of the two dogmas of the faith, namely, the Trinity and the Duality. It is to know and contemplate the three Persons of the Trinity as distinctively and indivisibly constituting the one God, and the divine and human natures of Christ as united in His single Person - that is to say, to know and profess that the single Son, both prior and subsequent to the Incarnation, is to be glorified in two natures, divine and human, and in two wills, divine and human, the one distinct from the other."
St. Gregory of Sinai, quoted in The Philokalia, Volume IV, compiled by St. Nikodimos of the Holy Mountain and St. Makarios of Corinth, translated and edited by G. E. H. Palmer, Philip Sherrard, and Kallistos Ware (Faber & Faber, 1995), p. 217.