The following is a brief selection of the Ante-Nicene (before Nicaea) testimony to this fundamental doctrine of the Christian faith, thus demonstrating that the doctrine of the Trinity is not a later invention of the Church at the Council of Nicaea (A.D. 325), as is frequently asserted by various internet apologists. While the formal articulation of the doctrine of the Trinity was certainly a later development as a result of responding to heretical teachings, what these quotations reveal is that the earliest disciples of Christ referred to Christ as God, recognized Him as being both God and man via the incarnation, and likewise asserted the Deity and personhood of the Holy Spirit. The distinction between the Persons of the Godhead is also maintained.
Ignatius: (Wrote in the early 2nd Century)
Author of the Epistle of Barnabas: (Written in the late 1st Century or early 2nd Century)
Polycarp: (Wrote in the early 2nd Century)
Hippolytus: (Wrote during the late 2nd Century to early 3rd Century)
Now that we have seen these Ante-Nicene articulations of Trinitarian theology, I turn your attention now to the more formalized Nicene Creed, finalized in AD 381:
 All quotations, except those of Hippolytus, are from The Apostolic Fathers: Greek Texts and English Translations, 3rd Edition, by Michael W. Holmes. The Hippolytus quotations are from Ante-Nicene Fathers, vol. 5.