The present volume is divided into two sections, ' Le Dieu Inconnu' and ' La Connaissance Mystique de Dieu '. In the first Festugière demonstrates, in my opinion convincingly, that the idea that God is incomprehensible and ineffable has a good Greek philosophical pedigree and is not, as Norden thought, contrary to the ' Greek spirit ' and the result of Oriental influence on later Greek thought. The chapter (v) in which he traces the idea back to Plato will no doubt be vigorously criticized by many Platonic scholars. My own impression is that he makes out a good case for his interpretation but would have done better to leave the Parmenides out of the argument (he does not rest too much weight on it). But even if his derivation of the doctrine from Plato is seriously challenged the main part of his argument remains intact. It cannot be denied that there are a number of passages in Plato which can be interpreted in this sense, and that they were so interpreted and that a doctrine of the transcendent First Principle beyond being, inaccessible to ordinary knowing and only attainable in rare flashes of intuition, was current among Platonists from a very early period in the history of the school; it seems now to be pretty well established that Speusippus held this doctrine (cf. Merlan, From Platonism to Neoplatonism 86-118, and the Speusippus fragment in the newly published Latin translation of the ' lost' end of Proclus, In Parmenidem, Plato Latinus III, 40). It was certainly common doctrine among the Platonists of the second century A.D., as Festugière shows in his next chapter, and he also makes clear that the Hermetic doctrine of the unknowableness of God does not differ from the Platonic.
In the second part of the book Festugière deals with great clarity and penetration, and a very commendable restraint about passing judgment on the experiences recorded, with the mystical knowledge of God according to the Hermetic writings. The first section, ' La Mystique par Extraversion,' includes an extremely valuable section on the development of the idea of Aion as a divine power in later Greek religion. Here again Festugiere tries to provide the idea with a wholly Greek pedigree ; but it does not seem easy to account for its development completely from Greek philosophical sources, and it does not seem to me impossible that it may have been influenced by the Persian Zrvan akarana, though I should agree with Festugière that no satisfactory proof of such an influence has been produced.
from rev. by A. H. Armstrong, Journal of Roman Studies, Vol. 45, Parts 1 and 2 (1955), pp. 188-189