Tertullian's polemic against a modalist teacher named Praxeas that works out, for the first time in Latin, the vocabulary of one substance and three Persons for Father, Son, and Spirit.
Adversus Praxean ("Against Praxeas") is Tertullian's treatise against a Christian teacher who taught that the Father himself became incarnate and suffered on the cross — the view later called modalism or patripassianism. Tertullian opens by accusing the devil of counterfeiting sound doctrine through Praxeas, who claims there is only a single Lord and therefore concludes that Father, Son, and Spirit must be the very same person appearing under different names. Against this, Tertullian sets out what he calls the oikonomia (the divine "economy" or arrangement): one God who has a Son, his Word, through whom all things were made, and a Spirit sent by both.
Over thirty-one chapters Tertullian works chapter by chapter through the case for real distinction within the Godhead without dividing its unity. He appeals to Wisdom literature, where personified Wisdom speaks of being begotten by God "at the outset of his undertakings"; to the invisibility of the Father set against the visibility of the Son who appeared to the patriarchs and prophets; and above all to the prologue of John's Gospel, where the Word is "present with God" and "was itself God" yet distinct from the one he is with. He answers Praxeas's favorite proof-texts — "I and the Father are one," "He who has seen me has seen the Father" — by insisting that unity of substance does not mean identity of person, coining the very formula that Nicene and later trinitarian theology would depend on: one essence, three Persons, distinct not in condition but in sequence.
Written early in the third century, Adversus Praxean is the single most consequential text in the emergence of Latin trinitarian vocabulary. Tertullian is the first author on record to use trinitas as a technical term and to distinguish una substantia (one substance) from tres personae (three Persons) — the exact framework the church would later formalize at Nicaea and Constantinople. The treatise also preserves, almost as a byproduct of refuting Praxeas, one of antiquity's clearest windows onto how ordinary "simple" believers of the second and third centuries actually understood God, and why they resisted a doctrine of real plurality even while confessing one God.
“He claims there is a single Lord, the almighty maker of the world, so that he can turn even this oneness into a heresy.”
— Adversus Praxean – Chapter 1
“believe in one God only, but under the following administration, which we call the oikonomia: that the one God also has a Son, his Word, who proceeded from him, by whom all things came to be and apart from whom nothing came into being”
— Adversus Praxean – Chapter 2
“as though he were not, in this very sense, still one — since all things come from one — through unity of substance, and the mystery of the oikonomia is nonetheless still preserved, which arranges the unity into a trinity, setting forth Father, Son, and Spirit as three”
— Adversus Praxean – Chapter 2
“They assume that the trinity's number and arrangement splits the unity apart”
— Adversus Praxean – Chapter 3
“At the origin there existed the Word, and that Word was present with God, and that Word was itself God; this one, from the origin, stood together with God”
— Adversus Praxean – Chapter 21
Praxeas was a Christian teacher, likely active in Rome, who taught that the Father himself became incarnate, suffered, and died as Jesus Christ — a position Tertullian treats as effectively erasing any real distinction between Father and Son. Little is known of him apart from what Tertullian reports, and some scholars have suspected Praxeas is partly a rhetorical stand-in for a broader modalist trend. Tertullian opens the treatise by charging that Praxeas 'did two works for the devil at Rome: he drove out prophecy and he brought in heresy; he put to flight the Paraclete and he crucified the Father.'
Modalism holds that Father, Son, and Spirit are not really distinct but are the same single divine person appearing under three different names or 'modes.' Patripassianism is the specific claim that the Father therefore suffered on the cross. Tertullian rejects this because it collapses the New Testament's own language of the Son praying to the Father, being sent by the Father, and being distinct from the Father into confusion, and because scripture repeatedly treats Father and Son as two who can address one another.
It is Tertullian's formula for holding unity and plurality together: Father, Son, and Spirit share a single divine substance or essence, but are distinct 'Persons' within that one essence, distinguished (in his words) 'not in condition but in sequence, not in essence but in appearance.' This vocabulary, coined here for a Latin-speaking audience, became the working grammar later councils used to formalize trinitarian doctrine.
He is the first extant Latin author to use trinitas as a settled technical term for the threefold Godhead, which is why Adversus Praxean is treated as a foundational document in the history of the word and the doctrine, even though the underlying ideas draw on earlier Greek writers such as the Apologists.
He leans heavily on Wisdom literature (Wisdom as a personified 'second Person' begotten by God before creation), on episodes where God is seen or not seen by figures like Moses (used to argue the visible God of the Old Testament is the Son, not the Father), and on John's Gospel prologue, where the Word is 'with God' and 'was God' yet distinguishable from the God he is with.
He argues the verse says 'one thing' (unum), not 'one person' (unus), so it expresses unity of substance and agreement, not identity of person — exactly the distinction his one-substance/three-Persons formula is built to capture.
Beyond refuting one teacher, it supplies the Latin theological vocabulary — substantia, persona, trinitas — that later Nicene and Chalcedonian formulations depend on, making it a hinge document between the informal, often modalist-leaning language of ordinary early Christians and the technical creeds of the fourth and fifth centuries.
It dates from Tertullian's later, Montanist-influenced period, generally placed in the first decades of the third century CE, after his split from the mainstream Roman church but before his death.
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