360. Now, though Plato had joined with an imagination the most splendid and magnificent, an intellect not less deep and clear, yet it is not to be supposed that either he or any other philosophers of Greece or the East had by the light of nature attained an adequate notion of the Holy Trinity; nor even that their imperfect notion, so far as it went, was exactly just; nor perhaps that those sublime hints, which dart forth like flashes of light in the midst of a profound darkness, were originally struck from the hard rock of human reason, but rather derived, at least in part, by a divine tradition (Sects. 298, 301), from the author of all things. It seems a remarkable confirmation of this, what Plotinus observes in his fifth Ennead, that this doctrine of a Trinity--Father, Mind, and Soul--was no late invention, but an ancient tenet.
361. Certain it is that the notion of a Trinity is to be found in the writings of many old heathen philosophers--that is to say, a notion of three divine Hypostases. Authority, Light, and Life did, to the eye of reason, plainly appear to support, pervade, and animate the mundane system or macrocosm. The same appeared in the microcosm, preserving soul and body, enlightening the mind, and moving the affections. And these were conceived to be necessary universal principles, co-existing and co-operating in such sort as never to exist asunder, but on the contrary to constitute one Sovereign of all things. And, indeed, how could power or authority avail or subsist without knowledge? Or either without life and action?
362. In the administration of all things, there is authority to establish, law to direct, and justice to execute. There is first the source of all perfection, or Fons Deitatis; secondly, the supreme reason, order, or λóγοσ ; and lastly, the spirit, which quickens and inspires. We are sprung from the Father, irradiated or enlightened by the Son, and moved by the Spirit. Certainly, that there is Father, Son, and Spirit; that these bear analogy to the sun, light, and heat; and are otherwise expressed by the terms Principle, Mind, and Soul, by One or τò ''εν, Intellect, and Life, by Good, Word, and Love; and that generation was not attributed to the second Hypostasis, the νοûς or λóγοσ, in respect of time (Sect. 352), but only in respect of origin and order, as an eternal necessary emanation; these are the express tenets of Platonists, Pythagoreans, Egyptians, and Chaldeans.
363. Though it may be well presumed there is nothing to be found on that sublime subject in human writings which doth not bear the sure signatures of humanity, yet it cannot be denied that several Fathers of the Church have thought fit to illustrate the Christian doctrine of the Holy Trinity by similitudes and expressions borrowed from the most eminent heathens, whom they conceived to have been no strangers to that mystery, as hath been plainly proved by Bessarion, Eugubinus, and Dr. Cudworth.
364. Therefore, how unphilosophical soever that doctrine may seem to many of the present age, yet it is certain that men of greatest fame and learning among the ancient philosophers held a Trinity in the Godhead. It must be owned that upon this point some later Platonists of the Gentile world seem to have bewildered themselves (as many Christians have also done) while they pursued the hints derived from their predecessors with too much curiosity.
This is all from Siris. The notion of a primitive revelation to all humanity is common in this period; one finds mention of it, for example, in Malebranche and Butler (as far as I know, Malebranche merely mentions it, but Butler defends it).