Wouldn't that then mean that this God isn't part of "existence"?
Things not part of existence, are called non-existant....
Just saying.
Talking about the "source of existence" seems inherently self-contradicting, since the source of existence would have to exist in order to be able to invoke it. So existence is its own source then.
Anytime anything is said about God or the Absolute, using dualistic language you will run into a contradiction. To even imagine God as "other" to anything else, is itself a contradiction. For instance, to say you believe God is Infinite, and then to imagine God as a "being" separate from your own being, denies God's infinite reality.
If taken literally, that God is outside creation, would make God's infiniteness more like a block of Swiss Cheese, with holes of "non-existence" in which you and I someone live "outside" of the Absolute. Even to use the word God at all, suggests its opposite of Not-God, which would make God a finite creature, instead of the Absolute.
So saying God is the Source of existence, should not be taken in strictly dualistic terms, even though we are using dualistic language, which is all we have available to us, frankly. God is nondual, and even if we using dualistic language, it is not truly being used or understood in dualistic terms. They are metaphors, not definitions when you are pointing to the Absolute.
That completely depends on how you wish to define this God.
In all cases though, one surely would have to leave behind scientific scepticism and rationale, because a belief in gods, or anything supernatural, is, as you surely know, scientifically unjustifyable..
I would not call it "unjustifiable" scientifically. Its nature is by definition, beyond what can be defined and examined through a scientific lens adequately. Science investigates rocks and planets and chemicals, and things like this. It doesn't delve into the interior spaces of human experience. It doesn't deal with the humanities. It doesn't deal with the whole of human life and experience. It is only one set of eyes, not the only set of eyes through which to see and understand reality. Science is not the measure of all that is real, and is not empowered to pass judgement on what is justifiable beyond its reaches.
Because some people just don't understand, or don't want to understand, that when your beliefs demonstrably don't agree with observable reality, it's not reality that is incorrect.
But why do they do this? Do they lack the faith necessary that allows their beliefs to be modified?