Tertullian, Against Praxeas
In his letter Against Praxeas, Tertullian defends the orthodox doctrine of the Trinity. He acknowledges that the heresy to be refuted is caused by Satan himself. The heresy of Praxeas, “He says that the Father Himself came down into the virgin, was Himself born of her. Himself suffered, indeed was himself Jesus Christ”. These “tares” of Praxeas, force Tertullian to both explain the church’s position on the doctrine, and secondly move him to deal with the misapprehensions of the opposing view.
The Church’s Position
First, there is The Church’s Position. There is only one God, but in the economy (i.e., the distinct roles each member of the triune Godhead fulfills) of the Godhead is the Son who proceeds from the Father, who created all things, who was sent into the virgin by the Father, and from the Father through the Son the Holy Spirit is sent. Tertullian asserts that this rule of faith is not new, but rather has been handed down to the church from its inception. The unity is one of substance (i.e., of nature—divine,), and the three-ness constitutes the persons Father, Son, and Spirit (i.e., one of identity—distinctions).
Second, there is Praxeas’ Objection. Although the following objection did not originate with Praxeas, the allegation raised against the church’s view of the Trinity, is that it leads people to either bi-theism (i.e., two Gods) or tri-theism (i.e., three Gods), whereas their view of God leads them to the true worship of the one God. Moreover, they assert that their view maintains the sole monarchy of God, whereas the church’s view destroys it. Tertullians’ essential response is that the unity of the monarchy is not destroyed, but rather it is preserved, if the Son and the Spirit are indeed sharers of the one monarchy.
Varied Responses to Heresy
Third, there is Tertullians’ Varied Responses to the Heresy. One response to the heresy is that the unity of the Godhead and the supremacy and sole government of the divine being are not impaired according to Catholic doctrine. Tertullian argues that since the Son is derived from the substance of the Father, does only the will of the Father, and is given all power from the Father, then the Monarchy is not destroyed from the faith. Moreover, since the Spirit proceeds from the Father through the Son, the monarchy ends up not being destroyed, but rather, it is preserved. Furthermore, the fact that the Son will restore the monarchy back to the Father, demonstrates the clear distinction of persons within the Godhead. Henceforth, those who are claiming to preserve the sole monarchy are actually destroying it, because they are overthrowing the very arrangement and dispensation employed by God.
Clarification of the Trinity
Fourth, Tertullian clarifies the Catholic rule of faith concerning the Trinity. He argues that the Father, Son, and Spirit are a unity of substance, but are three distinct persons. The Father is seen as the entire substance, the Son and the Spirit are derivations of that whole. The distinction of persons can be seen in that the Father begets, and the Son is begotten, and the Son sends another Paraclete. The distinction of persons is further seen in the names of Father, Son, and Spirit.
Monarchian Position not Coherent
Fifth, he shows the incoherence of the Monarchian position that maintains the Father is the Son and vice versa. He does this by distinguishing being from having. Tertullian argues that in order for a father to be one, he must first have a son. Likewise, in order for a son to be one, he must first have a father. Moreover, how can I be my own son, or be my own father? The logic is faulty, and yet the Monarchian responds with “nothing is impossible with God!” Tertullian’s challenge is to consider whether or not God has really done it. For he reasons that God really could have made man with wings to fly, but reality does not bear it out, nor does the Monarchian argument for that matter.
Scripture Must Ground Our Positions
Sixth, Tertullian then challenges Praxeas to biblically ground his position. He then distorts a passage to make his point concerning the distinction between the Father and Son, “The Lord said unto Himself, I am my own son, today I have begotten myself “. If this is the case, then God is a deceiver, an imposter, and a tamperer with His word. But since the contrary obtains, the position asserted by Praxeas is egregiously false.
Textual Evidence for Plurality of Persons
Seventh, he then demonstrates the scriptural basis for the plurality of persons (Gen.1: 3, 26-27; 3:22; Jn.1: 1, 3, 9), and the unity of substance within the Godhead as a remedy to combat polytheism (Ps.45: 6-7; Isa.45: 14-15; Jn. 1:1; etc.), and then chastises Praxeas for not accepting the clear declarations of scripture.
Further Evidence From Both OT and NT
Eigth, Tertullian continues with scripture passages in the OT (Gen. 32:30; Ex. 33:13, 11; Num. 12:6-8; 1 Cor.13: 12; Mk. 9:4; Mt. 17:3; etc.) and in the NT (Jn.1: 1-2, 18; 4:12; 1 Cor.9: 1; 1 Tim. 4:16; etc.) demonstrating the Fathers’ invisibility and the Sons’ visibility. Moreover, he deals with OT manifestations of Christ, with titles that both the Son and the Father share depicting their deity, and he abundantly shows how in Johns’ Gospel, the distinction of persons between the Father and Son obtain.
Tertullian not only sees that the doctrine of the Trinity is the great divide between Christianity and Judaism, but he also sees the Monarchian doctrine as blasphemous, and as such, damnable.
Many well-meaning professing believers today fall under the error of Praxeas punting to “nothing is impossible with God” God is “mysterious” and a host of other responses that undermine the clarity of Scripture concerning God’s nature and the distinction of persons within the Trinity. While mystery obtains (e.g., Christ’s incarnation) it’s the duty of disciples to not take the Name of the LORD our God in vain (i.e., misrepresenting His Character or Being).
The doctrine of the Trinity is in fact one of the pillars of Christendom distinguishing it from all other beliefs, and it is foundational to understanding so much of Scripture.
 Tertullian, “Against Praxeas,” Ante-Nicene Fathers, Volume III, Pp.597-627, (T & T Clark Edinburgh, Wm. B. Eerdmans Publishing Company, Reprinted in 1997)