The ante-Nicene fathers, from the second to the fourth century after Christ, taught and defended the doctrine of the Trinity. Below are two examples:
Tertullian (ca. 160–225)
The first defence of the doctrine of the Trinity was by Tertullian, who was born around 160 AD. He explicitly “defined” the Trinity as Father, Son, and Holy Spirit and defended his theology against Praxeas, although he noted that the majority of the believers in his day found issue with his doctrine.
Origen (ca. 186–255)
The God and Father, who holds the universe together, is superior to every being that exists, for he imparts to each one from his own existence that which each one is; the Son, being less than the Father, is superior to rational creatures alone (for he is second to the Father); the Holy Spirit is still less and dwells within the saints alone. So that in this way the power of the Father is greater than that of the Son and of the Holy Spirit, and that of the Son is more than that of the Holy Spirit… (Origen, First, 33–4 [I.3]) Origen had been excommunicated later on in the Second Council of Constantinople for regarding the Holy Spirit as less hypostasis.
Trinity doctrine had always been clearly observable in the Old Testament scriptures and the Jewish Rabbis interpretations, as well as in the New Testament scriptures and the early fathers’ writings.
The Trinity is traceable from Genesis to Revelation.