Why call him God?

Epicurus was an ancient Greek philosopher who founded the school of philosophy, "Epicureanism". 

He was a great thinker, and audacious to question the stereotypes and dogmas of his time. He even questioned God in a quaint manner.  

Theists and believers of God proudly claim that God is omnipresent, omnipotent, and omniscient. 

Epicurus posed mind-boggling questions about omnipotency of God.

Is God willing to prevent evil, but not able?

If God exists, and is willing to prevent evil, then what is stopping the omnipotent deity from exacting his duty on lowly humans. Does God require means to destroy evil; but that would be self-contradictory. Is God powerless or unable to purge evil from this world? If that is the case, God is certainly not omnipotent.

Is God able, but not willing? 

Granted God is omnipotent, and rules over everything under the sun - being the Creator, Designer and all, could it be He is not willing to prevent if not eradicate evil? If God himself isn't willing to pulverize evil, then He truly is menacing and malevolent. Anyone who has power and wants to do something, can easily do the part. When you see something bad, you must do something: this is common intuition. An adult cannot stand by watching a child being slaughtered, unless he is a maniac.
What God who calls himself merciful would want to see his creation in tatters? Only a sadist would enjoy the unsightly sight.

Is God both able, and willing?

Wait, then why isn't there any action? Could it be our sadist God loves delirious sabotage? Given that God can and wants to dispose of evil and malign in this world, then why is there so much evil in the world? If a supreme deity, that is God, controls the worldly affairs from his all-high Throne, why don't we see any observable or empirical results of his divine power? Simply, because God does not exist and all claims to his existence are groundless fairy tales - a load of crap.

Is God neither able, nor willing?

This one is pretty self-explanatory. If God has no divine power, nor does He want to obliterate evil, there is ultimately no reason to call such useless deity a God.