Many times we hear of the term "Trinity" first being used when Emperor Constantine I combined the Roman Church with the Empire, as a "new concept".
It is said that he based his decision on Tertullian, who was supposedly the first one to use the term "Trinity". Tertullian was about 200 A.D. The 3rd Century, A.D.
Evidence has now surfaced to date the usage of the term much earlier than 200 A.D., about a generation earlier.
In a letter from one Pastor of the early Assemblies, Theophilus to Autolycus: Book II, Chapter XV (115-181 A.D.), late 2nd Century A.D.
"But the moon wanes monthly, and in a manner dies, being a type of man; then it is born again, and is crescent, for a pattern of the future resurrection. In like manner also, the three days which were before the luminaries, are types of the Trinity, of God, and His Word (LOGOS), and His wisdom."
Eusebius further praised Theophilus as one of the Pastors of the early Assemblies who labored hard to protect his flock from heresy.
You will also notice that the quote from Theophilus is in a "midrashic style" and it was not stated by Eusebius that the Term was a "new concept", altogether foreign to the Assemblies, but the Term itself is not mentioned. Instead, Eusebius praised Theophilus for his work in guarding the flock. This may indicate that the concept, perhaps even the Term "Trinity" itself, may not have been considered some "new concept" in the Assemblies at all.
With Special Thanks to Pastor Buddy Martin for locating this early reference.