On that point, in relation to the New Testament, it's indisputably the case that it does not use words like
ousia (consubstantial, of one essence or being), persons or Trinity.
The first to use quasi-Trinity language as we know it today, was St. Theophilus of Antioch in 169 A.D.
Yet whilst the 'language' used is post-biblical, it is nevertheless still an attempt to articulate ideas that can be found in the New Testament but not yet 'phrased' in the philosophically sophisticated ontology of Greek and Latin scholars.
The New Testament authors were Jews relying upon Hebraic phraseology (in Greek, which Hellenistic Jews had been using for quite some time by then) to express these same basic, underlying concepts in the way that they had available to them.
However, I would say that the consensus of modern scholars is that the roots - in crude and primitive form (not yet expressed with the penetrating philosophical and technical acumen of the later Church Fathers) - of the later Trinity are already implicit in the New Testament, in the form of a
Binitarianism with a triadic discourse in relation to God.
Jesus is already cast in Pauline epistles like Corinthians and Romans, and in the Gospel of John and Epistle of the Hebrews, as the earthly incarnation of the pre-existent divine Word / Son, who always existed in relation to the Father in eternity, and through whom the universe was created.
And this Father - Son dynamic within the one reality of God, is always spoken of in relation to the
Spirit, even though the latter is not yet fully personified in the NT.
The 'Son' is referred to as the Image of God ("
who being the brightness of His glory and the express image of His being" (
Hebrews 1:3)) and also as 'begotten' (first-born, begotten means to be 'born') in a number of NT texts. “
Monogenes” is the Greek word used for “begotten” in
John 3:16,
John 3:18,
John 1:14,
John 1:18 and
1 John 4:9.
In every scriptural verse where John employs the word
MONOGENES, it is in a context in which he simultaneously relies on the term
GENNAO “new birth” (see
John 1:14,
18;
3:16,
18;
1 John 4:9). That is no accident. The Johannine author is intentionally making a distinction between the new birth that believers experience and the Son’s
unique begottenness from the Father.
(
John 1:14 YLT)
And the Word became flesh, and did tabernacle among us, and we beheld his glory, glory as of an only begotten of a father, full of grace and truth.
[Jhn 1:14 MGNT] (14) καὶ ὁ λόγος σὰρξ ἐγένετο καὶ ἐσκήνωσεν ἐν ἡμῖν καὶ ἐθεασάμεθα τὴν δόξαν αὐτοῦ δόξαν ὡς μονογενοῦς παρὰ πατρός πλήρης χάριτος καὶ ἀληθείας
"
The Son is the image [eikon] of the invisible God, the firstborn over all creation. For in Him all things were created" (
Colossians 1:15)
"
For to which of the angels did God ever say,
“You are my Son;
today I have begotten you”?" (
Hebrews 1:5)
So, the basic concept and even terminology of God being one, yet having within this one reality a relation of Father and Son, whereby the latter is the begotten self-image of the former and both are one Creator God, is already there.
And the latter theologians relied upon the exegesis of such verses to define Trinitarian theology at the councils.
Consider this from the
Epistle of Alexander, Bishop of Alexandria, to Alexander, Bishop of Constantinople, dated 324, the year before the Council of Nicaea, as an example of how they did so:
“
In their [referring to Arius and his followers] ignorance and want of practice in theology they do not realize how vast must be the distance between the Father who is unbegotten (ἀγεννήτος), and the creatures, whether rational or irrational, which He created out of the non-existent; and that the only-begotten nature (φύσις μονογενής) of Him who is the Word of God, by whom the Father created the universe out of the non-existent, standing, as it were, in the middle between the two, was begotten of the self-existent Father (ἐξ αὐτοῦ τοῦ ὄντος πατρὸς γεγέννηται), as the Lord Himself testified when He said, ‘Every one that loveth the Father, loveth the Son that is begotten of Him (τὸν υἱὸν τὸν ἐξ αὐτοῦ γεγεννημένον)’ [1 John 5:1]” (NPNF2 3.39 modified).