The article was written by well respected scholar James Dunn, and this is from an article in Explorations vol 8, Num 2, 1994. (bolds are mine for emphasis)
1. Monotheism.
The simple answer is that Christianity lost the clarity of a belief in God as one,
that is, one without further qualification.
Of course, Christianity continued to claim that it is and always has been a monotheistic faith. But there is no doubt that the doctrine of the Trinity, if it is to be adequately appreciated, requires well-informed and highly sophisticated powers of thought. So much so, that many Christians could be described as effective tri-theists in their actual understanding of the Trinity. Here, however, it is necessary to appreciate the extent to which earliest Christian thought of God was simply an extension of Jewish reflection on the same subject (divine Wisdom, apocalyptic and mystical vision, the Shekinah, etc.); and whether a more “straightforward” monotheism involves too much of an over-simplification of a subject unavoidably complex for merely human language.
The propriety of attributing (divine) pre-existence to other than God alone is not only a Christian question (the pre-existence of the Torah). The fundamental problem of how to conceptualize both the unknowable transcendence of God and, at one and the same time, God’s self-manifestation in the world was (and is) Jewish before it became Christian. Similarly the questions of how God may be known and of the status of the vehicles of God’s self revelation raise the same issues in both Judaism and Christianity. The function of Jesus within these complex questions may be more of a piece with Jewish monotheism than is at first apparent. Nevertheless, the contrast between the apparently simple clarity of Jewish (and Muslim) monotheism and the apparent complexity of Christian Trinitarianism remains a stumbling block and to Christianity’s detriment.
Since Nicaea ‘Son of God’ has all the overtones of the full blown Trinitarian formula ‘Son of God’ means second person of the Trinity, ‘true God from true God, begotten not made’, etc. In other words, our problem is the difficulty of hearing ‘son of God’ other than from this side of Nicaea. It often comes, therefore, as something of a shock to realize that it was not the same pre-Nicaea….At the time of Jesus ‘son of God’ was a way of characterizing someone who was thought to be commissioned by God or highly favoured by God. It was not necessarily a title of divinity.
