That's a totally different statement than "God is a myth". A myth is "a story that informs the ethics and morals of a people". Myths can be historically generally regarded as historical, like George Washington Crossing the Deleware. Or generally regarded as not historical, like George Washington Chopping Down the Cherry Tree. Both are myths of national origin.

So God is most definitely a myth. A myth that became fact most historically evident in the Resurrection of Christ. Start there - prove that the eye witness testimonies of the resurrection are or are not historical. Then you can argue about what that event or non-event means.