Trinitarians of course say that the Trinity is throughout scripture, however, if you ask a Jew about the Old Testament and how the Hebrews defined God, there’s no question but that they would say there is only one God.
No doubt, Trinitarians support the doctrine of the Trinity with scripture. The question isn’t about whether or not this can be done, but whether or not Trinitarian theology was on anyone’s mind, including Jesus’, or the disciples’, or the apostles’ as Christian theology was emerging as reflected in New Testament scripture as it was being written; much of it from oral tradition.
Given as an example, Jesus’ teachings were a departure from the common Jewish understanding of the law, of Messiah, and admonishments of the conduct of Jewish leaders. Of course because it was controversial, Jesus’ ideas and commentary are clearly laid out. We have a very good historical note in time when those ideas originated and why there was a need to clarify distinctions between old and new ideas.
We have the very same marker for the Trinity, and it is exactly in the fourth century as a dispute between Arianism and Trinitarians; though a complete meaning of “Trinity” wasn’t yet defined in some “final language”.
The question as to whether or not the Trinity is an idea held by early Christians, starting with Jesus himself, is quite easy to answer. That answer is “no”, unequivocally. Were Jesus, the disciples, or the apostles to have at all implied an ontology of God different to the Jewish conception of God, the doctrine would appear in some clear way even if not in some “final language”; just as when the idea did emerge evidenced in no other earlier writings than Tertullian, finalized in 385 CE.
The absence of any controversy between Jesus and early Christians and the Jews damningly means all agreed on God’s ontology. More, that when the Jews began to think Jesus had a different ontology about God and in particular, whether or not Jesus saw himself as a literal son of God, Jesus said “No! I am not God, but the son of God in the very same way you are: a prophet, a teacher, anointed, burdened with those responsibilities” (referencing Psalms and giving its definition of what a “son of God” meant). Jesus’ response, mind you, was exactly what kept him from being stoned.
The absence of any conversation about the ontology of God or any commentary in the New Testament remotely suggesting Trinitarian theology or any attention at all to how the Father, the Son, and the Holy Spirit relate to one another, and the complete absence of an ontology of Jesus aside from his own reckoning with Psalms, or an ontology for the Holy Spirit ought to clearly indicate nothing new was being said at all about who and what God is.
There has only been a controversy about the Trinity since that idea emerged, which is clearly in the fourth century. And in the twenty first century, the origin of the basis of the idea of the Trinity ought not be a controversy. We know it is not in the New Testament. We know that no patristic thinker before Tertullian spoke of such a thing. We know when a controversy over the ontology of God arose.
As all Christian theology is diverse and developmental, it is completely appropriate to adopt novel ideas. However, we must acknowledge they are new developments. We must also confess that they cannot be ideas which must be believed in order to be Christian, lest we are wont to say Jesus too was a Trinitarian, or a Predestinationalist, or a Dispensationalist, and so on, or that perhaps James or Peter were not Christians. In that case, the only sense in which these ideas, the Trinity in particular here, define any centrality about Christianity is that for some community professing to follow Christ, those ideas are central to that community’s way of thinking about a Jewish carpenter from Galilee.