Friedrich Schleiermacher; the great 'Father of Modern Theology', at the end of his magnum opus, The Christian Faith, insightfully identified the 'being of God in Christ and in the Christian Church' as the main pivot for the ecclesiastical doctrine of the Trinity, only to go on to state that in his judgement they could stand independent of it. No evaluation before or since better captures both the necessity and ambiguity of the doctrine. It is the perspective of this essay that the Christian doctrine of God requires a confession of the Holy Trinity.
Clearly, though, one cannot separate the Christian understanding of God as Trinity from the religious monotheism of its closest two relatives, both of whom along with Christianity lay claim to Abraham as their ancestor. The conviction that the one unsurpassable God is self-revelatory in word and deed is the common testimony of Judaism, Christianity and Islam.