Steven Avery's claim is quite simply wrong.
Tertullian makes absolutely no connection whatsoever with John's First Epistle, in situ, in Adversus Praxeam Chapter 25.1 (which is what this thread is about).
It's clearer than clear when read in situ.
Tertullian show's everyone exactly where he draws his
EIS-EGESIS from.
Tertullian of Carthage
Adversus Praxeam
Translated By Peter Holmes
Chapter 25
"What follows Philip's question [John 14:8], and the Lord's whole treatment of it [i.e. Phillips question], to the end of John's Gospel, continues to furnish us with statements of the same kind, distinguishing the Father and the Son, with the properties of each. Then there is the Paraclete or Comforter, also, which He promises to pray for to the Father [John 14:16], and to send from heaven after He had ascended to the Father. He is called "another Comforter,"[John 14:16] indeed; but in what way He is another we have already shown, [i.e. in Adv. Prax. Chapter 13] "He shall receive of mine," [John 16:14] says Christ, just as Christ Himself received of the Father's." (Emphasis, note, some verse refs, added by me)
Tertullian of Carthage
Adversus Praxeam
My Translation
Chapter 25 (continued)
"In this manner the connection of the Father in the Son, and of the Son in the Paraclete is accomplished, three [masculine identities] that are connected together, each one originating from out of the other. But how these three are [Var. "sint" = "But how these three might be"] one [neuter thing], not a single [masculine identity], is simply as [Or: "is just as"] it says "I and the Father, we are one," [John 10:30] by the uniting of the substance(s) [Latin = plural], not by any singularity of number(s)"
Though Steven desire's it, assert's it vehemently, and repeat's it etc, Steven Avery is mistaking Tertullian's own words (his own
EIS-EGESIS) for an interpolation that did not exist in Scripture yet (a kind amphibological fallacy).
The grammatical nature of Tertullian's argument requires that his words "qui tres unum sunt, non unus" etc, be translated more explicitly into English than what would be required in either 1 John 5:7 and 8 Clause's D.
Tertullian simply reiterates the same grammatical argument which he made more explicit in a previous chapter.