Sam Harris and C.S. Lewis were not neuroscientists nor were they involved with any large field study of consciousness. So, how, from all of that, do you come up with "therefore, God!" To make that positive claim requires evidence or you have to humble yourself to admitting you simply don't know.
Nobody actually knows the probability of our universe existing. Because we have to ask
compared to what? We don't know for certain what the status of our universe was prior to the Big Bang( if the BBT is true) or if our universe sort of always existed. It's very hard to speculate without sufficient evidence to use as an anchor to reality. Mathematics doesn't exist actually. It's an abstract concept human invented for understanding nature. However, math is consistent and its universality is derived from the stable cause and effect properties of reality. This makes math a great descriptive tool we can apply to reality. However, math can be perfectly consistent and not have anything to do with reality. Think of Ptolemy's circles within circles theory of the solar system.
I think the argument you're trying to put forward here is that since the universe
appears to be so fine tuned that we can exist, which is an enormous amount of complexity, then only a divine being can be the answer. Is this at all accurate?
If it is I have a couple objections and maybe you'll find them of some value.
1) There's no
actual evidence for any of these claims. Just speculation and crafty argumentation.
2) Almost every part of the observable universe is hostile to mankind. What I mean is that if the universe was specifically fine tuned for us (life) by some grand supernatural designer then why can we only sustain in this infinitesimally small spec of silicate matter called Earth and pretty much no where else in the observable universe without massive
man made technological assistance? Wouldn't a well designed universe be more accommodating to well designed creation?
3) If a supernatural designer is required to account for all this complexity because something complex requires a designer, then it would stand to reason that any designer is more complex than that which it designed.But if the claim is something very complex requires a designer, and the designer is even more complex than that, then, by logical induction, what designed the designer?
(TL;DR for 3, the argument is shit is complex, complex shit needs a designer...so , the designer himself is complex and if complex shit needs a designer , who the fuck designed the designer?

... I added the graphic language for effect)
In all instances it seems like "God" is a non-answer.
I don't know if you're accurately typed. I don't know you.