The first unambiguous Triune God reference to be found in Christian writings appears during the latter 2nd Century, in the works of
Theophilus of Antioch, though the concept presented is not the same as the contemporary Trinity. Never the less, the treatment strongly indicates the idea was not new, nor original to Theophilus.
By the late 4th or early 5th Century, the Trinity concept almost as known today was a central, if perhaps not yet dogmatic, article of faith, proclaimed in what has become known as the Trinitarian Creed, the
QUICUNQUE VULT (for its opening words in Latin - though most scholars believe it most likely originally was written in Greek), or, most commonly, the
Athanasian Creed, but which whatever its origin almost certainly was not - could not have been - written by early Church Father
Athanasius.
Athanasius, a participant in the
Nicean Councils of the early 4th Century, was however a staunch proponent of
Homoousion, the principle that God the Father and God The Son in the person of Jesus Christ were "of one substance". Largely through Athanasius' efforts, this principle was incorporated into the
Nicene Creed, arguably a foundational document for what has become today's Christianity. In the late 6th Century, with the adoption into the Nicene Creed of the
Filioque Clause and its being finally "set in stone" nearly 500 years later by Pope Leo IX (occasioning
The Great Schism ), (discussed also
Here and
Here ), the contemporary Trinity became a dogmatic article of Christian faith.