Our reason seems to convince us that something must exist as “all that is”. Otherwise what would exist beyond it? So, reason concludes that something exist as the infinite or “all that is”. It cannot be anything objective because objects are limited by boundary. It can only be intelligence and awareness. And since we observe about us a highly evolved orderly universe we conclude it to be a higher intelligence that is sustaining all we experience and call IT God.
Having concluded that God is infinite, we can further conclude that He/It is all. There can be nothing else separate from It because if anything existed separate, one would infringe upon the other and no infinite could be possible.
Consequently, there can be no separation. God is all and all is God. All creation, including us and our experiences is God expressing.