Most Trinitarians are completely ignorant of the history of the Trinity and how what is being called by that term has changed over time. And this is wilfull ignorance. I will now quote from Tertullian’s book Against Praxeas chapter 3, from the Evans 1948 translation:
For all the simple people, that I say not the thoughtless and ignorant (who are always the majority of the faithful), since the Rule of the Faith itself brings over from the many gods of the world to the one only true God, not understanding that while they must believe in one only yet they must believe in him along with his economy, shy at the economy. They claim that the plurality and ordinance of trinity is a division of unity – although a unity which derives from itself a trinity is not destroyed but administered by it. And so put it about that by us two or even three are preached, while they, they claim, are worshippers of one God – as though unity irrationally summed up did not make heresy and trinity rationally counted out constitute truth. “We hold”, they say, “to the monarchy”: and even Latins so expressively frame the sound, and in so masterly a fashion, that you would think they understood monarchy as well as they pronounce it: but while Latins are intent to shout out “monarchy”, even Greeks refuse to understand the economy. But if I have gathered any small knowledge of both languages, I know that monarchy indicates neither more nor less than a single and sole empire, yet that monarchy because it belongs to one man does not for that reason make a standing rule that he whose it is may not have a son or must have made himself his own son or may not administer his monarchy by the agency of whom he will. Nay more, I say that no kingdom is in such a sense one man’s own, in such a sense single, in such a sense a monarchy, as not to be administered also through those other closely related persons whom it has provided for itself as officers and if moreover he whose the monarchy is has a son, it is not ipso facto divided, does not cease to be a monarchy, if the son also is assumed as partner in it, but it continues to belong in first instance to him by whom it is passed on to the son: and so long as it is his, that continues to be a monarchy which is jointly held by two who are so closely united.
In other words, the Father is the King. He has a human Son to whom he has shared his reign. And an angelic being, the Holy Spirit, with whom also he shares it. But only the Father is actually God. The others are only “God” in the sense of sharing in the economy or administration of God. This is the Trinity that Tertullian believed in, the first Trinity doctrine, circa 210 AD.
Ok, actually Tertullian does claim the Son is derived from the Father’s substance, and the Holy Spirit is derived from the substance of the Father through the Son, but he still clearly doesn’t mean it exactly like Trinitarians today. And even if he did, we have his admission that the MAJORITY of Christians rejected his view in” his day, both of Latins and Greeks, because they held to the “monarchy” of God rather than the “administration” or “economy.”
What’s my point? Well, not sure I have one per se. Oh, yes, here it is: Christians have not always believed in the Trinity and don’t let any Trinitarian tell you that we have. And don’t let any Trinitarian tell you that you ain’t a Christian if you don’t believe it today. And don’t let them tell you Tertullian believed in the same Trinity they are pushing, because he didn’t. By the way, to read the whole chapter, see Against Praxeas on tertullian.org run by the illustrious Roger Pearse.