[Over the next week you can expect a large number of posts as I summarise and process all my reading on the 4th century; sorry for clogging your RSS readers, and for the rest of you hope you find something useful]
Tertullian stands as the source and root of the Latin theological tradition. He is the first major significant Latin theological writer, engages in a vigorous defence of the faith against modalistic monarchianism, and his influence on Cyprian, Novatian, and the later Latins, is clearly evident.
When we come to read Tertullian, I believe we need to extend a certain generosity, and be prepared to read him against his context, and not against the standards of a post-381 orthodoxy. There are points at which he poorly expresses himself, lacking a developed pro-Nicene vocabulary, and there are points at which we may further question his theology itself, but we should not conflate the two.
Against Praxeas is Tertullian engaging and critiquing monarchianistic views of the Trinity, Praxeas himself probably being a modalist, and emerging no earlier than 208 AD. This work reveals some of Tertullian's key moves, emphases, and exegesis.
It is interesting to reflect that Tertullian, with his opponents, assumes the unity of God. The locus of the debate is going to be the distinction of the Persons, and the accusation of his opponents will be a tendency to ditheism. This is a recurring feature that I will note again in later theologians.
With that said, Tertullian presents a monarchia of God that is grounded in the Father and unthreatened by the Son or Spirit. He views the Son as from the Father's substance, doing nothing without the Father's will, and as having received all power from the Father (4).
Then Tertullian begins to speak about the Logos with God in the beginning. He seems to develop a picture of the Word as with God as an internal accompaniment of Reason (5), which is made equivalent to Wisdom (6), which is likewise always with God. In light of latter developments, Tertullian's Logos-Christology is deficient.
By (8) though Tertullian is speaking of the Son as prolatio from the Father. Though specifically disavowing a Gnostic (Valentinian) usage, and insisting on the Son's inseparability from the Father, the form of words available to Tertullian lets him down here. Perhaps we can detect the struggle with Latin, and especially North African, early Christianity with overly-literal readings of the scripture, and material conceptions of divinity.
Yet Tertullian is quite capable of stating some rather foundational Trinitarian principles. So, (9) the Father, Son and Spirit “inseparable from each other”; “the Father is one, and the Son one, and the Spirit one, and that They are distinct from Each Other”; This does not imply separation for “it is not by way of diversity that the Son differs from the Father, but by distribution...not by division...but by distinction” and “they differ one from the other in the mode of their being”.
Tertullian now moves to counter specifically Monarchian/Modalist claims (11f), with exegesis both of their texts and his own counter-texts. His main aim is to preserve the distinction of Father and Son, without implying ditheism (so in 13 he treats Ps 45:6-7, Isa 45:14-15, Jn 1:1 for this end).
From 14 onwards, Tertullian makes a move that from a later perspective seems ill-founded. He identifies the Father as invisible, and the Son by contrast visible. Though he conceives of the pre-incarnate Son's visibility as different to his incarnate visibility, he nonetheless maintains it. He then treats OT theophanies as Christophanies. The fundamental problem is this: invisibility is a property of the divinity, not the person of the Father, for God is Spirit, incorporeal, immaterial. To ascribe visibility to the Son goes too far, and is the same kind of step Aetius and Eunomius will make with regards to unbegotten/begotten, to show the Son's essential unlikeness to the Father.
In 18, Tertullian picks up his game a little; treating passages that speak of God 'alone', he establishes the principle that whenever one Person is mentioned, the others can be inferred; secondly, that the uniqueness of God is spoken in contexts that are polemical of idolatry and polytheism, not the divinity and distinction of the Son.
From 27 onwards, Tertullian shifts focus to speak of the humanity and divinity of Christ; Praxeas names God as Spirit and Christ, the Son as flesh and man. Tertullian states instead “We see plainly the twofold state, which is not confounded, but conjoined in One Person – Jesus, God and Man” and “the property of each nature is so wholly preserved”. He then uses this principle to say that Christ died in his humanity (29), against the Monarchianist implication of Patripassianism, that the Father died on the Cross. He complements this (30) by asking what sense it makes for the Father to abandon himself on the Cross? No, only the distinction of Father and Son will preserve this.
While I have highlighted what I think are some key problems in Tertullian's Against Praxeas, we should recognise his considerable achievement. Tertullian defends a significantly developed account of distinction and unity within the Godhead, without either a technical vocabulary or a long theological tradition of the same. His exegesis, the visibility issue aside, is well-placed to refute modalist conceptions, and his treatment of the two natures of Christ is well-founded and polemically accurate. While much of the details of his work are not taken up by later authors, his general contribution and influence is an important foundation for later Latin theologians.