“Then is God perfectly simple and true both in word and deed; he changes not; he deceives not, either by sign or word, by dream or waking vision.” (Plato, The Republic, Bk. 2)
We need not suppose that Plato and other pagans who posited some version of simplicity in the divine being got it from the Holy Scriptures. That was a notion floating around in the Middle Ages for a while. Their error does not remove from our inquiry how it is that Plato knew of divine simplicity. Certain Greeks came to see that despite the pervasiveness of polytheism some approach toward monotheism would be requisite when reasoning from natural things, lesser to greater. Multiplying gods (ie., physicalist theories of causation personified as semi-divine beings) was not the ultimate answer to their perplexity about the one cause of all causes. The answer to “the causes of things” was not in the diversitas but in the unitas.
Neither is supposing that later theologians *imposed* simplicity onto their reading of Scripture. The Hellenization Thesis has been sufficiently sent to pasture. As an aside it is more likely that there was a historical Christianization of the hellenization of Mediterranean societies (see Tom Holland, Dominion).
So, we should, on the other hand, see that Plato and others took simplicity to be an imperative truth to the divine being because they saw that truth without the aid of Scripture. In other words, it should be obvious to the reasoning observer that simplicity is a first stage, so to speak, trait of divine being when reasoning from natural reason. In fact, not only is it a first stage observation. Simplicity is a first order doctrine or principle to the divine being. Then after that, the next step is seeing that simplicity reasoned from natural reason is further confirmed in the rationally deduced and spiritual truth of divine simplicity made manifest in the proper Name of God, ehyeh “I am” or YHWH Qui est, “He who is” (Exodus 3:12-18). Contemplating the holiness of God via divine simplicity is a devotional and theological practice which the divine Name gives basic and ordinary means.