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