I’m bad at intros, so I’m just going to jump right into it. This is an explanation of a theory I have that, in order for a solipsist to accept that the external world exists, he must believe in God. Now this doesn’t have to be Yahweh, or any god in particular, it merely needs to serve the function that I am presenting.
If reality cannot be proven to be more than a construct of consciousness, then in order for the external world to exist independently of my consciousness, it must exist, in full, in some consciousness that is at a higher level than mine. In this case, I am a figment of this higher consciousness’s imagination, as are all the people and beings that I encounter.
Another way to think of it: If I accept that the external world exists, then I must accept the existence of other minds, other minds that, like mine, cannot prove the existence of anything but their own minds. So unless we are all actively constructing the universe collectively, than there is a mind outside the universe imagining the entirety of it. Now there may be evidence of the former if we look at quantum mechanics in this light. If we are all actively constructing the universe as we go, then the more we probe into the inner workings of the universe, the more we have to actively construct. So you could see the anomalies and contradictions of quantum mechanics as our minds not being able to accurately construct all levels of the universe. Since subatomic particles behave differently when they are being observed, this theory is entirely possible. But if the latter is true, then we are simply pushing the bounds of what we can understand being figments of the imagined universe, and only God can fully understand it, if that.
This is not an original concept. The illustration above represents one version of Hindu cosmology in which Brahma, sitting on a lotus flower that grows out of the navel of Vishnu, who rides on the back of a serpent in a primordial endless ocean (just ignore that part for now), dreams the universe. Vedantic Hinduism claims that Brahma is all things, and that the universe we live in is Brahma’s dream, and the Atman (the individual self) is a manifestation of Brahma in his own dream. Hinduism may be one of the oldest religions known to man, not to mention the oldest one still practiced today, yet even here we find the concept of solipsism giving rise to God (or Brahma).