Chrysorrhoas

The feast of St. John Damascene, Doctor of the Church, is today. His name at birth was probably (although not certainly) Yanah ibn Mansur ibn Sarjun, and he was a Christian in Muslim-occupied Syria in the eighth century. His family worked for the civil service under the Umayyad caliph, as they had under the Byzantine emperor before the conquest, and so, in turn, John did too, until he left for the monastery. His most famous work in the West is the Exposition of the Orthodox Faith. From Book I, Chapter V:

The Deity is perfect, and without blemish in goodness, and wisdom, and power, without beginning, without end, everlasting, uncircumscribed, and in short, perfect in all things. Should we say, then, that there are many Gods, we must recognise difference among the many. For if there is no difference among them, they are one rather than many. But if there is difference among them, what becomes of the perfectness? For that which comes short of perfection, whether it be in goodness, or power, or wisdom, or time, or place, could not be God. But it is this very identity in all respects that shews that the Deity is one and not many.

Again, if there are many Gods, how can one maintain that God is uncircumscribed? For where the one would be, the other could not be.

Further, how could the world be governed by many and saved from dissolution and destruction, while strife is seen to rage between the rulers? For difference introduces strife. And if any one should say that each rules over a part, what of that which established this order and gave to each his particular realm? For this would the rather be God. Therefore, God is one, perfect, uncircumscribed, maker of the universe, and its preserver and governor, exceeding and preceding all perfection.

Moreover, it is a natural necessity that duality should originate in unity.