One God

Who said in ancient Greece believed in many gods? The allegorical significance of Twelve Gods Greek faith had already begun to emerge from the philosophical origins and writing, the Presocratic philosophers who define the gods's analogy with the four natural elements (fire, air, earth, water), for example Hera with air or Zeus with fire.
Aristotle saying and complained for Xenophanes (the teacher of Parmenides), that he is not clarified if this One God who proclaim is immaterial (such as the God of Parmenides) or material (such as the God of Melissos), and attaches to him that "with the eye pointing to the whole Heaven (Universe) Xenophanes says that the One is God." For Xenophanes has been stating that he was the first to introduce the true monotheism that was ever on earth (according to Wilamowitz).

Influenced by Xenophanes, Plato in Timaeus scripture explains the creation of the one Cosmos (Universe) from the Father, the perfect one God. During the following centuries the Greek, the Christians or the medieval or Arab philosophers mentioned the last Plato's writing to consolidate the philosophy of religion. The uniqueness and existence of God accepted by Aristotle or downstream fans even beyond the Middle Ages.
The Platonist divine Plutarch, influenced by the Pythagorean philosophy, quotes the opinion of his teacher Ammonius for the one God, in "declaring the “E” at Delphi." This textbook is perhaps from the most important of which relied many Theologists after Plato's "Timaius", perhaps St. Augustine as-well. According to "E" explaination at Delphi (XVII) entrance meaning "only you exists", like an acceptance to One God that is the only certain existing entity, simple and autonomous, one eternal and perfect, because everything else is subject to birth and decay as the time itself , all represent this World.
But this World should be perfect because if it were not, then the creator would also not be perfect hence would not be God as not perfect. However the creator is one or many? He should be one because if they were many, they should were all perfect in order to be Gods hence identical, hence one, while if they were not identical then should be someone more perfect, hence again one perfect God while remainder as not absolutely perfect they would not be gods.

In the precedents appears as acceptance the existence of God. However is there  any proof? Aristotle gives Cosmological and teleological approach and many others also considering the God as axiomatic faith. Claiming that the creator does not exist, it should exist the comprehension of the creator in order to be true this statement, otherwise this proposition is false. However is in effect and the opposite that leads to the not existence. Therefore here we reach in the segregation of human "faith" or "glory" from the true knowledge, with the last one will be, for the same reason, one. According to Plato for something we think or dream it should be somewhere, in some place to be exists. Adding to this if it is someone’s thought could be fallacy, if it is a thought of infinite men then it can be a proof. 
But how can we understand God ? We can approach only God because of our being imperfect. Therefor our world is one, the best that God created from all His possibilities, but different from "His World" who is full from endless ideas (icons or archetypes, "ta katholou"), according to Plato. But how our one world created and what is real if everything is changing, so it is not exist? The origin is Logos, the Word or Reason or Alethea the one real truth of Parmenides, the father of Philosophy. Logos is govern everything, introduced by Herakletus and Logos became latter true in Christ, the God's humanized will in our Cosmos.
He has made from one blood every nation of men to dwell on all the face of the earth, and has determined their preappointed times and the boundaries of their dwellings, according to St. Paul. (Acts 17:26, speaking to Athenians for the Unknown God).