It is a consistent assertion of many critics of biblical Christianity that Christians before the Council of Nicea in 325 A.D. did not believe in the doctrine of the Trinity. Conspiracy theories abound where this council (or Constantine, the Roman Emperor of that time) supposedly invented the doctrine and forced it on all the churches. Some (like the Jehovah's Witnesses) go even further. They claim that that the Council of Nicea itself did not affirm the Trinity, and that is was invented and forced on the church at an even later date. They argue that the Nicene Creed is not clearly Trinitarian because it does not emphasize the Holy Spirit enough for their satisfaction. The creed actually does assert belief in the Holy Spirit alongside belief in the Father and Son after going into great detail on the full deity, unity, and distinction of both the Father and the Son. Further, the context of Nicea was a controversy over the nature of the Son rather than the Spirit, so the excessive emphasis on the one rather than the other makes perfect sense in context, but that is not enough to satisfy such critics. Whatever the variation of this myth, the common insistence is that the Trinity was developed by some late church council or some nefarious pagan emperor who hijacked Christianity.
The assumption seems to be that if Christians were not yet using specific formulas of the doctrine of the Trinity like we have today, then that must mean that no one at that time believed in such a doctrine. It's apparently not enough for writers to have talked about or alluded to the doctrine naturally in the course of normal discussions, they have to have laid it out in a single, isolated statement or the doctrine wasn't there. This standard is rarely, if ever, applied to any other doctrine. Such an insistence is, in fact, fallacious. One can read the early church writings for themselves and see how often the various aspects of the doctrine of the Trinity are openly expressed or plainly assumed by many of those writers, even if they did not repeat a concise formula and lay out the doctrine in a single paragraph all in one place.1 Still, it is also worth noting that the doctrine of Trinity was laid out in exactly the kind of brief formula such critics demand in the writings of at least one early church leader before the time of Constantine or the church councils. Between 260-270 A.D. (over fifty years before the council of Nicea and, in fact, before Emperor Constantine was even born) Gregory Thaumaturgus wrote in his brief work, “A Declaration of Faith”:
“There is one God, the Father of the living Word, who is His subsistent Wisdom and Power and Eternal Image: perfect Begetter of the perfect Begotten, Father of the only-begotten Son. There is one Lord, Only of the Only, God of God, Image and Likeness of Deity, Efficient Word, Wisdom comprehensive of the constitution of all things, and Power formative of the whole creation, true Son of true Father, Invisible of Invisible, and Incorruptible of Incorruptible, and Immortal of Immortal and Eternal of Eternal. And there is One Holy Spirit, having His subsistence from God, and being made manifest by the Son, to wit to men: Image of the Son, Perfect Image of the Perfect; Life, the Cause of the living; Holy Fount; Sanctity, the Supplier, or Leader, of Sanctification; in whom is manifested God the Father, who is above all and in all, and God the Son, who is through all. There is a perfect Trinity, in glory and eternity and sovereignty, neither divided nor estranged. Wherefore there is nothing either created or in servitude in the Trinity; nor anything superinduced, as if at some former period it was non-existent, and at some later period it was introduced. And thus neither was the Son ever wanting to the Father, nor the Spirit to the Son; but without variation and without change, the same Trinity abideth ever.” 2
As one can see here, the Father, Son, and Spirit are rightly distinguished from one another and placed in eternal relation to one another. They are also declared to be one perfect, undivided, eternal being; uncreated and unchanging. The Doctrine of the Trinity was thus plainly articulated clearly, systematically, and concisely by Christians before the Council of Nicea or any possible influence of Emperor Constantine.