If the unity and multiplicity of things are governed by their being, and if the entire being of things depends on God, as has been shown to be the case the cause of plurality in things must be sought in God. How this comes about, must now be examined.
Any active cause must produce its like, so far as this is possible. The things produced by God could not be endowed with a likeness of the divine goodness in the simplicity in which that goodness is found in God. Hence what is one and simple in God had to be represented in the produced things in a variety of dissimilar ways. There had to be diversity in the things produced by God, in order that the divine perfection might in some fashion be imitated in the variety found in things.
Furthermore, whatever is caused is finite, since only God’s essence is infinite, as was demonstrated above. The finite is rendered more perfect by the addition of other elements. Hence it was better to have diversity in created things, and thus to have good objects in greater number, than to have but a single kind of beings produced by God. For the best cause appropriately produces the best effects. Therefore it was fitting for God to produce variety in things.
St. Thomas Aquinas. (1265-1274). Compendium Theologiae: Cause of Diversity, trans. by Cyril Vollert. St. Louis & London: B. Herder Book Co., 1947
All scripture is given by inspiration of God and is profitable for doctrine, reproof, correction, and instruction in righteousness (2 Timothy 3:16).