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This page won't make much sense if you didn't come here from The Trinity: Development, Doctrine, and Definition.
Tertullian is considered the height of orthodoxy on the matter of the Trinity. As you have seen, however, he believed that the Son was begotten of God in eternity past, in some manner that is peculiar to God. He also believed that before that birth, which happened before anything else was created, that God was alone.
The modern version of the Trinity, as found in the Athanasian Creed and invented in the 4th century, doesn't really have room for this. "The Father is eternal; the Son is eternal; the Holy Spirit is eternal; yet there are not three Eternals, but one."
But Tertullian's orthodoxy is never questioned.
Origen, on the other hand, writing just a couple decades after Tertullian, is often castigated as unorthodox, if not downright heretical. He was censured by a Church council a couple centuries after he died.
Origen was an unusual thinker, and he did have some wild ideas. His views on the Trinity, however, were not among them.
Origen was actually closer to the modern view of the Trinity than Tertullian was. Origen loved to think about things like eternity and time. To Origen, if the Son was begotten in eternity past, then he was always begotten. There was no "time" when the Son was not yet begotten, nor a "time" when God was alone.
Origen, as I said, had other wild ideas, so his writings come under closer scrutiny than Tertullian's. Though they taught the same things about the Trinity, it is Origen whose orthodoxy is questioned. Tertullian, who could be quoted in exactly the same ways, is left alone.
And no one seems to notice that Origen was simply teaching the definition of the Trinity common to the Church in his day. In fact, no one seems to notice that Origen teaches the view espoused by the Nicene Creed.
All the while we quote the Nicene (and Apostles) Creed, never knowing that we neither understand nor believe its doctrine of the Trinity—and condemn Origen with it!