"God is an abstract entity by any sensible definition"
Not according to most theists.
That's where you're wrong! Can you consider the possibility that you don't understand what theism is?
(gee, I'm starting to sound like Luis Youdon'tunderstandtime Hamburgh)
Ask them if they think a God physically exists, and they will say 'yes.'
You are assuming all theists are idiots (not that some of them aren't...)
The major reason people believe in God is because they can only understand things in terms of cause and effect. That poses a dilemma for the existence of everything, because while some things cause others to exist, you can't possibly regress that ad infinitum and still understand it. So a first cause is assumed to exist, and we call that God. Sure, we also attribute human properties to that, but if you think about it, it does make sense. Most people are not stupid, they are just not good at expressing themselves.
Now before you go bickering about the fact that the concept of God does not solve the causality problem, and it really doesn't, let me warn you that if you do talk about that you are missing the point.
The concrete existence of God is at the heart of almost all theism I've encountered, your peculiar quasi-solipsistic theism notwithstanding
First, I'm neither a solipsist nor a quasi-solipsist. If you think I am, that means you don't understand what I'm talking about. Let me give you an example of what you often seem to misunderstand:
Have you ever seen the color red? If you say yes, I'll say you are lying. You may have seen red objects, but the concept 'red' does not exist apart from 'red objects'. There is no such "thing" as 'the color red', 'red' as a standalone "thing" is just an idea in your head. And the same goes for "objects". Have you ever seen an "object"?
You call that solipsism, I call it seeing things as they truly are. I'm not saying red objects don't exist, all I'm saying is "stuff exists", some of that stuff we call "red objects", and if you think of "red" and "object" as independent entities which exist by themselves I say you are the solipsist, you are the one giving actual existence to abstract entities. As soon as you label something, and start refering to the label as if it were the thing being labeled, you have left the domain of reality and stepped into the domain of your mind. The true solipsist is the person who believes that labels exist. A person such as yourself.
Back to the "concrete existence of God". God is a principle underlying all things, and as such it cannot exist independently of "all things". Or can it? I maintain the question is as meaningless as "would the color red exist if red things didn't exist?". It's not that the answer must be a negative (or positive), the real issue here is that the question shouldn't be posed in the first place.
So the concrete existence of God is associated with the concrete existence of the universe. But don't ask Joe on the street about that, his understanding of theism is too simplistic for Mario the intellectual inquisitor. Joe is a practical guy and he takes from religion that which matters to him and leaves the details for the experts. Joe also takes from science that which matters and also leaves the details for the experts.
I hope you keep in mind that if you charge people with accepting religion without first solving its philosophical challenges, you must be as critical of their blind acceptance of science.
Now that took you by surprise huh? :-)