Wherefore this very ancient opinion [δόξα] comes down from theologians [θεολόγων] and from lawgivers to poets and philosophers; it can be traced to no source, but it carried a strong and almost indelible conviction, and is in circulation in many places among barbarians and Greeks alike, not only in speech and tradition but also in mystery cults [τελεταῖς] and sacrifices, to the effect that the Universe is not of itself suspended aloft without sense or reason or guidance, nor is there one Reason which rules and guides it by rudders, as it were, or by controlling reins, but, inasmuch as Nature brings, in this life of ours, many existences [οὔθ᾽ εἷς ἐστιν] in which both evil and good are commingled, or better, to put it very simply, Nature brings nothing which is not combined with something else, we may assert that it is not one keeper of two great vases who, after the manner of a barmaid, deals out to us our failures and successes in mixture, but it has come about, as the result of two opposed principles and two antagonistic forces, one of which guides us along a straight course to the right, while the other turns us aside and backward, that our life is complex, and so also is the universe; and if this is not true of the whole of it, yet it is true that this terrestrial universe, including its moon as well, is irregular and variable and subject to all manner of changes. For nothing comes into being without a cause [εἰ γὰρ οὐδὲν ἀναιτίως πέφυκε γίγνεσθαι], and if the good cannot provide a cause for evil, then it follows that Nature must have in herself the source and origin of evil, just as she contains the source and origin of good..--Plutarch, On Isis and Osiris, 45.
My Ghent colleague, the historian Peter Van Nuffelen, discusses the passage quoted from Plutarch in his wonderful Rethinking the Gods: Philosophical Readings of Religion in the Post-Hellenistic Period (62).Van Nuffelen calls attention to the fact that Plutarch does not dismiss ancient opinion (δόξα understood here "to indicate the non-philosophical character of primitive wisdom), but rather construes it as an instance of ancient wisdom shared by many peoples and introduced by theologians and lawgivers into human society. The truth is, in Van Nuffelen's felicitous phrase, "couched in symbols and myth." (62) It's the task of the philosophers to unearth the wisdom of ancients in the laws and maxims, but also in "mystery cults and sacrifices." The Plutarchian philosopher is, thus, not just a cultural and political historian, but also a kind of anthropologist capable of interpreting a variety of practices and 'going native' among the mystery cults. (Plutarch himself was a priest at the famous oracle of Delphi.)
As Van Nuffelen makes clear, relying on "philosophy" as his "guide," Plutarch reads "his own ideas into specific cults." (63 n. 90) In the quoted passage (and this goes unmentioned by Van Nuffelen), Plutarch insists that a version of the Principle of Sufficient Reason (i.e., nothing comes into being without a cause) can be found among the variety of expressions of ancient wisdom. In particular, the original version of the PSR inclines not to Monism (as we might expect post Spinoza-Leibniz), but rather to a Zoroastic Dualism with two opposed powers that constitute the world because* all existence always has a positive or negative valence. It is no surprise, then, that Plutarch names Zarathustra among the wisest [σοφωτάτοις], who teach such opinions (as well as superstitious myths (μυθολογία)) and his era is situated before the dawn of historical time, "five thousand years before the time of the Trojan War."**
On Plutarch's account, religion and law have chronological precedence over poetry, which goes back to Homer, and philosophy, which is of more recent invention (Van Nuffelen 63). In particular, one important example, of the wisdom of the ancients is introduced into law and religion together by the "wise and just" (σοφίᾳ καὶ δικαιοσύνῃ) Numa, who, we are repeatedly told, legislated by way of persuasion [πείθων]. In fact, Numa is an innovator, that is, the source of a re-founding that orients Rome away from constant war to reasoned peace.
In my opinion, Plutarch models his Numa, who makes sure that before he accepts Roman power that there is a sign of the gods in his favor, on the precepts of the Athenian Stranger in Plato's Laws (as Plutarch hints); Numa, after all, rules by persuasion and religious awe.+++ As others have noted, Plutarch's near contemporary, Philo had treated Moses as a wise, lawgiver-religious innovator, in similar fashion (see here and here; see also Carlos Fraenkel). Spinoza (perhaps inspired by Machiavelli) reads Moses in very similar fashion, but disagreeing with Philo, treats (recall) Moses as a itself an exemplar re-deployed by Ezra in the narrative accompanying his re-founding. In fact, while Spinoza may not have been a friend of ancient wisdom, he certainly thought that the start of philosophy, however confusedly, is available everywhere and at all times in human feeling.
*Plutarch hints at the idea that this dualism might be local to the terrestrial earth and not all-pervasive in the universe.
** As Va Nuffelen notes (67), Zoroaster is listed together with Numa among the constitutional founders.