Tomus ad Antiochenos

Tomus ad Antiochenos is a letter or mediation proposal written by Bishop Athanasius of Alexandria on behalf of a regional synod he convened in Alexandria in 362, addressed to a group of bishops seeking a solution to the schism between "Eustathians" and "Meletians" in the parishes of Antioch. This letter played a key role in the Trinitarian theological debates between the one-hypostasis model and the three-hypostasis model of the Trinity, anticipating the turning point in this question from the 370s onward.^[1]

The central concern is to achieve theological agreement based on the Nicene Creed. By recognizing that certain theological points of contention in the Arian controversy were based not only on differences of belief, but also on different language rules or conceptual differences between Latin and Greek, the Tomus paved the way for the Trinitarian theological language rules of the three Cappadocians from the 370s onward: Basil of Caesarea, his brother Gregory of Nyssa, and their mutual friend Gregory of Nazianzus subsequently established the concept of the one being (Ousia) and the three hypostases of God, which became binding with the decision of the First Council of Constantinople in 381.