Letter 50063: The mystery of the Trinity is the foundation of everything we believe, and I want to set it out as clearly as I can.

From: Ambrose, Bishop of Milan To: The Church at Milan Date: ~381 AD Context: An alternate recension of a doctrinal letter explaining the Trinity to the Milanese congregation, likely delivered as a sermon and then circulated in written form. Ambrose, Bishop, to the faithful. The mystery of the Trinity is the foundation of everything we believe, and I want to set it out as clearly as I can. We confess one God in three persons: Father, Son, and Holy Spirit. Not three Gods — that is polytheism. Not one person appearing in three modes — that is the heresy of Sabellius [a third-century theologian who taught that Father, Son, and Spirit were merely three masks or modes of one person, a doctrine called modalism]. But one God who is, in his eternal nature, a communion of three distinct persons sharing one divine substance. "How can one be three and three be one?" the skeptic asks. My answer: I do not know how. I know that it is. The question "how" belongs to mathematics. The question "that" belongs to revelation. God has revealed himself as Trinity — in the baptism of Jesus, where the Father speaks, the Son is baptized, and the Spirit descends (Matthew 3:16-17); in the Great Commission, where we baptize in the name of Father, Son, and Spirit (Matthew 28:19); in the whole arc of salvation, where the Father plans, the Son accomplishes, and the Spirit applies. The Arians say the Son is less than the Father. We say: then baptism is a fraud, because you are baptizing in the name of a God and a creature. The Macedonians say the Spirit is less than the Son. We say: then the Spirit who dwells in you is not God, and you have no assurance that his indwelling actually saves. The stakes are not academic. If the Trinity is wrong, everything is wrong — baptism, the Eucharist, prayer, salvation itself. Get the Trinity right and everything else follows. Get it wrong and nothing holds. This is the faith of Nicaea. This is the faith of the apostles. This is the faith that will endure when every heresy has been forgotten. Hold fast to it.

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