St. Gregory the Theologian, Oration 38, Section 7:
God always was and is and will be, or rather always “is,” for “was” and “will be” belong to our divided time and transitory nature; but he is always “he who is,” and he gave himself this name when he consulted with Moses on the mountain. For holding everything together in himself, he possesses being, neither beginning nor ending. He is like a kind of boundless and limitless sea of being, surpassing all thought and time and nature. He is only sketched by the mind, and this in a very indistinct and mediocre way, not from things pertaining to himself but from things around him. Impressions are gathered from here and there into one particular representation of the truth, which flees before it is grasped and escapes before it is understood. It illumines the directive faculty in us, when indeed we have been purified, and its appearance is like swift bolt of lightening that does not remain. It seems to me that insofar as it is graspable, the divine draws [us] toward itself, for what is completely ungraspable is unhoped for and unsought. Yet one wonders at the ungraspable, and one desires more intensely the object of wonder, and being desired it purifies, and purifying it makes deiform, and with those who have become such he converses as with those close to him, — I speak with vehement boldness — God is united with gods, and he is thus known, perhaps as much as he already knows those who are known to him.
For the divine is without limits and difficult to contemplate, and this alone is entirely graspable in it, namely that it is without limits, whether one supposes that to be a simple nature is to be wholly ungraspable or perfectly graspable. For what is a being whose nature is simple? Let us inquire further, for simplicity is clearly not the nature of this being, just as composition alone is clearly not the nature of composite entities.