Under what circumstances would you say it is permissible to say some imagined entity was non-existent? What is actual evidence of absence? Or would you like to leave open the door for the possibility of Zeus? Ares? Odin? How about the Loch Ness Monster?
The stories of Zeus, Aries and Odin are just that. Stories. The same can be said about a huge chunk of the Old Testament of the bible. Most of the books contained are stories about God.
So if there are stories about Zeus, and stories about Yahweh, what is it that makes one's existence more plausible than the other?
There are stories about Christopher Columbus as well. What makes his previous existence plausible?