There was a time when the life of men was without order and like that of the beasts, subject to the rule of strength, an there was no reward for the good or any punishment for evil men. And then, I think, men set up laws for punishment, so the justice would rule and violence would be her slave. And if someone were to do wrong they would be punished. Then, since the laws prevented them from doing violent deeds openly, they continued to do them in secret.
I think that then some sound and clever-minded man invented fear of the gods for mortals, so that evil people would have some fear, even if they were acting or saying or thinking something in secret. Thereupon he introduced the divine being, saying: “There is a divinity, endowed with eternal life, who with his mind hears and sees and understands and attends to these things, bearing a divine nature, who will hear everything that is said amongst mortals, and be able to see everything that is done. If ever you plan some evil in silence, you will not escape the notice of the gods. For they are able to keep that in mind.”
Speaking these words, he introduced the most pleasant of lessons, concealing the truth with false speaking. He then claimed that the gods lived where he would terrify people the most. He knew the origins of mortals’ fears as well as benefits for their wretched life: from the revolving sky above, where they knew there was lightning and there were terrible rumblings of thunder. Around mortals he set up such fears, through which this man, by his words, nobly established the divinity in an auspicious spot, and he extinguished lawlessness with laws. Thus, I think, someone first persuaded mortals to believe that there is a race of gods.
Critias, Sextus Empiricus