Who is God? John of Damascus answers:
Therefore, we believe in one God: one principle, without beginning, uncreated, unbegotten, indestructible and immortal, eternal, unlimited, uncircumscribed, unbounded, infinite in power, simple, uncompounded, incorporeal, unchanging, unaffected, unchangeable, inalterate, invisible, source of goodness and justice, light intellectual and inaccessible; power which no measure can give any idea of but which is measured only by His own will, for He can do all things whatsoever He pleases; maker of all things both visible and invisible, holding together all things and conserving them, provider for all, governing and dominating and ruling over all in unending and immortal reign; without contradiction, filling all things, contained by nothing, but Himself containing all things, being their conserver and first possessor; pervading all substances without being defiled, removed far beyond all things and every substance as being supersubstantial and surpassing all, supereminently divine and good and replete; appointing all the principalities and orders, set above every principality and order, above essence and life and speech and concept; light itself and goodness and being insofar as having neither being nor anything else that is from any other; the very source of being for all things that are, of life to the living, of speech to the articulate, and the cause of good things for all; knowing all things before they begin to be; one substance, one godhead, one virtue, one will, one operation, one principality, one power, one domination, one kingdom; known in three perfect Persons and adored with one adoration, believed in and worshiped by every rational creature, united without confusion and distinct without separation, which is beyond understanding. We believe in Father and Son and Holy Ghost in whom we have been baptized.
And why would this God create?
Because of the exceedingly great wealth of His goodness, the good, all-good, and exceedingly good God, who is all goodness, did not rest content that the Good, or His nature, should just be and not be shared by anything. For this reason, He first made the spiritual and heavenly powers, and then the visible and sensible world, and then, finally, man of the spiritual and the sensible. Hence, all things He has made participate in His goodness by the fact that they have being. For He is being to them all, since ‘in him are all things,’ not only because He has brought them from nothing into being, but because it is by His operation that all things He made are kept in existence and held together. Living things, however, participate more abundantly, because they participate in the good both by their being and by their living. But rational beings, while they participate in the good in the aforementioned ways, do so still more by their very rationality. For they are in a way more akin to Him, even though He is, of course, immeasurably superior.
John of Damascus, De Fide Orthodox I:8, IV:13.