[Replied after some of those below]

Here, as below, I will confine myself mainly to Tertullian’s Against Praxeas and Gerald Bray’s Creeds, Councils, and Christ.

You assert that the oneness of God must be understood in an absolute unity rather than a composite unity (see The Hebrew language and Messianic prophecies). And Tom Wright pointed out, “there is no suggestion that “monotheism” or praying the Shema, had anything to do with the numerical analysis of the inner being of Israel’s god himself,” (more at The Incarnation: Why did God become Man?). In fact, to show that “there is nothing new under the sun”, Tertullian also noted the dogmatism of those who thought that belief in one God entailed belief in one Person in the Godhead:

this heresy, which supposes itself to possess the pure truth, in thinking that one cannot believe in One Only God in any other way than by saying that the Father, the Son, and the Holy Ghost are the very selfsame Person. As if in this way also one were not All, in that All are of One, by unity (that is) of substance; while the mystery of the dispensation is still guarded, which distributes the Unity into a Trinity, placing in their order the three Persons—the Father, the Son, and the Holy Ghost: three, however, not in condition, but in degree; not in substance, but in form; not in power, but in aspect; yet of one substance, and of one condition, and of one power, inasmuch as He is one God, from whom these degrees and forms and aspects are reckoned, under the name of the Father, and of the Son, and of the Holy Ghost. How they are susceptible of number without division, will be shown as our treatise proceeds.

Your other claim is based on something lost in the translations between Greek and Latin. Tertullian defined the Trinity as three Personae and one Substantia. As pointed out below, persona in Roman Law was a party to a legal contract. But a hyper-literal translation to Greek results in πρόσωπον prosōpon, which means “mask”. Thus the Greek church misunderstood the Latin usage, and originally thought that the Latin Church taught modalism. And the Greek Church formulated the Trinity as three hypostases (singular hypostasis ὑπόστᾰσις) and one oὐσία ousia. But a hyper-literal translation to Latin results in substantia—both sub and hypo mean “below”. Yet this is a misunderstanding, because in Greek theological usage, hypostasis meant a quasi-personification of attributes proper to a deity. Indeed, as you inadvertently note, it was used in the Greek New Testament to mean “person”.

Thus as Bray notes, in the 4th century, Basil (a noted creationist theologian) realized that what the Greek church meant by hypostasis, the Latin church meant by persona, so they really believed the same thing.

We are glad you liked the article. We can’t cover everything, but indeed, the originator of the Trinity is usually considered to be Tertullian (AD c. 160 – c. 225).

An instructive work is Creeds, Councils and Christ: Did the early Christians misrepresent Jesus? (updated 2009) by Gerald Bray, professor at Beeson Divinity School at Samford University in Birmingham, Alabama. He explains that Tertullian, a lawyer and Christian apologist, realized that the Bible taught that God made a covenant with Israel. And on God’s side, there were actually three signatories: the Father, Son, and Holy Spirit. Now in Roman law, the word for party to a legal action was persona. From this, Tertullian summarized the biblical teaching as tres Personae, una Substantia (three Persons, one Substance).

Tertullian explained the Trinity at length using copious biblical passages as proof that there was one God in three distinct Persons in Against Praxeas newadvent.org/fathers/0317.htm:

Bear always in mind that this is the rule of faith which I profess; by it I testify that the Father, and the Son, and the Spirit are inseparable from each other, and so will you know in what sense this is said. Now, observe, my assertion is that the Father is one, and the Son one, and the Spirit one, and that They are distinct from Each Other.

Praxeas was a heretic who taught a modalistic view like that of modern ‘Oneness’ groups: that the Father, Son and Holy Spirit were all the same Person in different modes. Tertullian convincingly demonstrates the error with Scripture, while Praxeas’ few Scriptures alleged to support that view are shown to do the opposite.

Actually, a little before Tertullian, Theophilus of Antioch (AD 115–181) wrote in an apologetic work to the learned pagan magistrate Autolycus. In a commentary on the fourth day of creation, Theophilus asserted that the previous three days were literal days before the sun, and “types of the Trinity, of God, and His Word, and His wisdom.” (To Autolycus 2:15, AD 181 newadvent.org/fathers/02042.htm).

The first statement is false: the Holy Spirit overshadowed Mary so she would conceive Jesus. As shown in The Incarnation: Why did God become Man?, this meant adding human nature to the divine nature that the Son already possessed from eternity. As Tertullian pointed out in Against Praxeas (see above):

Besides, the flesh is not God, so that it could not have been said concerning it, That Holy Thing shall be called the Son of God, but only that Divine Being who was born in the flesh, of whom the psalm also says, Since God became man in the midst of it, and established it by the will of the Father. Now what Divine Person was born in it? The Word, and the Spirit which became incarnate with the Word by the will of the Father. The Word, therefore, is incarnate; and this must be the point of our inquiry: How the Word became flesh—whether it was by having been transfigured, as it were, in the flesh, or by having really clothed Himself in flesh. Certainly it was by a real clothing of Himself in flesh. …

Of them Jesus consists—Man, of the flesh; of the Spirit, God—and the angel designated Him as the Son of God, Luke 1:35 in respect of that nature, in which He was Spirit, reserving for the flesh the appellation Son of Man. In like manner, again, the apostle calls Him the Mediator between God and Men, 1 Timothy 2:5 and so affirmed His participation of both substances.

Your second argument is just an argument from silence. The role of the Holy Spirit is largely to point people to Christ, not to Himself. It is folly to use such arguments and ignore the clear teachings of the personality of the Holy Spirit. For example, “the Holy Spirit said, ‘Set apart for me Barnabas and Saul for the work to which I have called them,’” (Acts 13:2), which shows the Holy Spirit referring to Himself in the first person.

Your third argument is the reverse of the truth. The early church was adamant that the true teachings must be derived from the Bible; we can see this copiously demonstrated by Tertullian, for example. And they fought strongly against pagan philosophies. Indeed, as Gerald Bray noted in Creeds, Councils and Christ (see above), “it looks strongly as if Platonism was refashioned to meet the challenge of Christianity, not the other way round.”