There is an apocryphal yet instructive story of a famous theologian who had just delivered a lecture on the doctrine of the Holy Trinity. A student jumped to his feet to thank the professor, excitedly exclaiming that at last he felt he really understood the doctrine. His theological teacher surprised him by heaving a sigh of despair and saying: “If you understand it as clearly as that, then you have misunderstood it and I shall have to start all over again.”
To speak today of God as three persons readily leads to a mental picture of a heavenly trio (a divine committee!), one which has even been portrayed visually in art. Such a view of God (and it is widespread in popular Christianity) deserves the condemnation which Muhammad heaped upon it. He called Christians polytheists, who had sadly regressed from the pure monotheism of Judaism which he himself felt called to reaffirm.
Lloyd Geering, February 28, 1987 issue of the New Zealand Listener
The full article from which the above quotes were taken can be read at Sir Lloyd Geering: Rethinking the trinity