In the fifth edition of his Christian Theology, the Anglican theologian Alister McGrath writes:
Alister McGrath - Christian Theology, Part 3, Chapter 10
The doctrine of the Trinity can be regarded as the outcome of a process of sustained and critical reflection on the pattern of divine activity revealed in Scripture, and continued in Christian experience. This is not to say that Scripture contains a doctrine of the Trinity; rather, Scripture bears witness to a God who demands to be understood in a trinitarian manner. We shall explore the evolution of the doctrine and its distinctive vocabulary in what follows.
Above, Alister McGrath frames the development of the doctrine of the Trinity as the inevitable outcome of the Bible's teaching about God, even though, as he acknowledges elsewhere, citing John Richardson Illingworth, the doctrine "did not attain its finally explicit formulation till the fourth century" (Christian Theology, Part 3, Chapter 10).
This hypothesis seems to suggest that the raw contents of the New Testament, although they demanded a Trinitarian doctrine to be rightly understood, were in some sense too stunning or perplexing for the first generations of Christians. It was only after "a process of sustained and critical reflection" that anyone was able to articulate exactly what Monotheism had become, after the advent of Jesus Christ.
There are a few different ways to understand this type of Trinitarian development hypothesis:
The first option above seems to be closer to what McGrath is arguing for in this book, but this article will address both, because both are heard commonly in debates around the subject of the Trinity and Deity of Christ.
Addressing the first possibility - that the New Testament authors simply did not know what category to place Jesus or the Holy Spirit into - an examination of the New Testament demonstrates this not to be the case. They clearly have a category for Jesus - the exalted Messiah, the "one Lord" under the "one God" (1 Corinthians 8:6). This pattern is repeated, explicit, and consistent throughout the New Testament. They positively identify Him as a figure other than the one God, and place Him under God (1 Corinthians 11:3, etc.).
Additionally, a narrative that would suggest that the Biblical authors had no category to put figures like Jesus or the Holy Spirit into does not really comport with human nature. When a person learns of a figure, they mentally place them into a category - a man, an angel, God, etc. If they have no category, they make a new one. If Jesus is not positively in the category of "Almighty God" for the New Testament authors, then Trinitarianism would represent a departure from the Biblical doctrine of God.
However, if one were to argue that the New Testament authors did positively conceive of Jesus as Almighty God, and yet could not articulate exactly what they believed - option 2 above - then other problems arise. Firstly, consider whether this is what is seen in the New Testament, or even the first few centuries of Christian writers. Do any of these authors plainly and unambiguously write that there are three entities who they believe to be described as Almighty God, and then lament over the fact that they cannot yet understand how such a thing is possible? The New Testament authors, in particular, are not confused, nor do they seem perplexed, or at a loss to describe what they think about God. An analysis of the New Testament usage of the term for "God" demonstrates that they think it is a co-referring term for the one called the "Father", who is "Yahweh" in the Old Testament.
Additionally, this hypothesis also runs contrary to human nature, in the sense that it suggests a very unnatural way of behaving for many generations of the first Christians. The belief that Jesus is Almighty God is very easy to state, and comprehend. So, one would expect such a thing to be very clearly stated, many times, from the earliest days of Christianity, if any of them actually believed that. Equally, the belief that the Father is God is very easy to state, and comprehend. And the same goes for the belief that Jesus is not the Father.
However, it does not take centuries of reflection to be presented with those three propositions, and realize that something needs to be explained, as they are apparently incoherent when grouped together. Very quickly, a rather easy-to-describe controversy would arise over how to understand these three basic claims, if they were all held and taught. That process would not take centuries, it would take mere days, or weeks. The writings of the New Testament would be almost entirely consumed with addressing the frequent questions of rather ordinary people, who would be puzzling over, or critiquing these three claims in some way. Yet, there is absolutely no trace of such a controversy in the New Testament - neither from believers, nor from critics.
That is to say, the doctrine of the Trinity - at least its basic constituent elements - would have been very easy to express, if anyone wanted to do so, right from the beginning. It does not take even a week of reflection to articulate, "The Father is God, the Son is God, the Son is not the Father, and I do not understand how". If such a belief had been there from the beginning, it would be stated clearly, and would have caused massive controversy. If these were present, this development hypothesis would make sense, and would have strong explanatory power. The fact that there is not even one such statement, or any trace of controversy, is an indication that this development hypothesis is actually illegitimate, despite it being a common attempt to explain why the vital doctrine of the Trinity appears so late in history.
If the New Testament authors believed Jesus was Almighty God, it would have been made very apparent. Because it is not apparent, going into their writings, and attempting to portray them as somewhat confusedly saying things which terminate in them believing that Jesus is Almighty God, though it was unclear to them, is actually just reframing (or even twisting) their writings according the whims of what later centuries of professional theologians would attempt to read into them.
That is to say, believers do not need to conceive of the New Testament authors as stunned or perplexed with Jesus to the extent that they were unable to articulate what they believed about Him and His relation to God. The writers of the New Testament have distinct categories for God and Jesus, and are not unclear in their exclusion of Jesus from the category of "Almighty God", which is reserved for the Father alone (John 17:3: 1 Timothy 2:5: 1 Corinthians 8:5-6, Ephesians 4:4-6).
Additionally, for those who would suggest that the New Testament authors believed that all three Persons of the Trinity were Almighty God, and yet were somehow unable to completely articulate or even describe the doctrine, leaving it as a task for later generations, it should be noted that the basic elements of the doctrine of the Trinity are actually very easy to state, and would have been stated by the New Testament authors if they had believed them, even if the details were not filled in. The fact that they do not make a point to do so in any formal way, and that there is no controversy to that end mentioned in Scripture, is a strong argument for Unitarianism. In this way, what is popularly presented as a slow and gradual march toward the truth can be viewed as a slow and gradual departure from what the New Testament teaches about God and His Christ.