St. Athanasius of Alexandria (ca. 296-373), who became bishop of Alexandria in 328, wrote the treatise Oratio conta gentes (Against the Heathen) to refute polytheism. In the following excerpt, he explains how the unity of God is proven by the harmony we observe in the created world.
“Since then, there is everywhere not disorder but order, proportion and not disproportion, not disarray but arrangement, and that in an order perfectly harmonious, we needs must infer and be led to perceive the Master that put together and compacted all things, and produced harmony in them. For though He be not seen with the eyes, yet from the order and harmony of things contrary it is possible to perceive their Ruler, Arranger, and King.”
“If we saw a city, consisting of many and diverse people, great and small, rich and poor, old and young, male and female, in an orderly condition, and its inhabitants, while different from one another, yet at unity among themselves, and not the rich set against the poor, the great against the small, nor the young against the old, but all at peace in the enjoyment of equal rights,—if we saw this, the inference surely follows that the presence of a ruler enforces concord, even if we do not see him; . . . so in the order and harmony of the Universe, we needs must perceive God the governor of it all, and that He is one and not many.”
“This order of its arrangement, and the concordant harmony of all things, shows that the Word, its Ruler and Governor, is not many, but One. For if there were more than one Ruler of Creation, such a universal order would not be maintained, but all things would fall into confusion because of their plurality, each one biasing the whole to his own will, and striving with the other.”
“Just as though one were to hear from a distance a lyre, composed of many diverse strings, . . . and would not fail to perceive from this that the lyre was not playing itself, nor even being struck by more persons than one, but that there was one musician, even if he did not see him, who by his skill combined the sound of each string into the tuneful symphony; so, the order of the whole universe being perfectly harmonious, and there being no strife of the higher against the lower or the lower against the higher, and all things making up one order, it is consistent to think that the Ruler and King of all Creation is one and not many, Who by His own light illumines and gives movement to all.” (38)
Quotations from A Select Library of the Nicene and Post-Nicene Fathers of the Christian Church, Second Series, Vol. IV, ed. Philip Schaff and Henry Wace (Edinburgh: T&T Clark, 1886).