In Plato’s Euthyphro, Socrates asks whether it is better to say that a thing is holy because the gods love it, or that the gods love it because it is holy. Socrates clearly favors the second option. To use “because the gods love it” (or, implicitly, “because it is God’s will”) as an unexplained explainer is to assert a form of theological voluntarism. As Leibniz said much later, this is to make of God a tyrant or a despot, arbitrary rather than wise and good.
It turns out that assertions of the form “it is God’s will” necessarily involve argument from authority. The exchange between Socrates and Euthyphro exhibits the hollowness and nonrational character of argument from authority in general.
Elsewhere, Plato famously makes Socrates argue that only the wisest should rule. The best rule is based on wisdom; only the worst is based on sheer power. (See also Freedom and Free Will.)
It might seem that to say that the gods love a thing because it is holy is implicitly to presume that what is holy is somehow simply given. Leibniz may or may not have presumed this, but I think Plato did not in regard to anything practical, because I don’t think Plato regarded anything in becoming as simply given.