"God is everything" is as valid a definition as any, and it has the advantage that this god demonstrably exists. However, it raises the question of why there should be a separate word for "God". Why not simply say "the universe"?
As much as it is blown out of proportion or context, pantheists such as these use the term 'God' as a metaphor for the universe or the unexplained areas of physics. Some quotes have been misinterpreted possibly deliberately and are still used in forms of the Argument from authority, such as the famous Einstein quote containing "...only then will we know the true nature of God...". When Einstein said this, he did not mean it in a religious way, but he meant 'God' as a metaphor for the unexplained areas of Physics.