It is not unfounded. Theists are simply doing what scientists do- applying the observed logical constants of our known existence to greater things. The time and place before our universe could be something that works in no way similar to our known universe, but for the sake of honest philosophical discussion, theists believe it is necessary to work within the realms of our known existence if we ever hope to understand our origins of existence.
This is not a philosophical discussion. It's a science discussion. But since we're talking about fairy tales, it's treated as if it were a matter of how you feel about it. You are not applying the scientific method at all. You are taking extremely limited information and reaching far flung conclusions. Applying what we know about space and time to things that happened before space and time existed is like arguing that the sun can't be too hot because it will melt. You are applying human scale and causality to the entire universe. Electrons don't even operate that way. There are particles in the universe that might be in multiple places at the same time. Not only does outside the universe not have to conform to our scale, other parts of this universe don't.
There is no evidence whatsoever to support the application of cause and effect on a human scale to the universe as a whole.
Sure, we can believe there was some alternate state of existence before us that defies everything we could ever know, both scientifically and philosophically, but it wouldn't really get us anywhere in reasonable debate and require just about as much faith as religion.
See, what you're calling "belief" is actually just ignorance. Instead, we have actual information about how the universe operated in its first few seconds and what implications that might have towards its beginning. We also have higher level mathematics that can describe universes that don't function the same way ours does, and can provide insight into how extra-universal things might operate. But we don't even assume that any of these hypotheses are true at this point. There's just not enough information. So we go with the one that is the closest to our observable data, until we find a better one that is even closer. We do not argue that it
has to be any known theory. It probably isn't.
Tell me about this "before" you speak of.
If you were paying attention, you'd understand that time is a factor of mass and energy. Neither of which necessarily exist outside this universe. Therefore, before and after, and thus cause and effect, do not necessarily govern anything outside this universe. There is no evidence to show that it does. So we cannot assume that it does.
We're arguing that within the confines of known existence, at our current time, it has to be god (that is, have the properties of God). It is plausibly true to believe that everything in our physical realm of existence has a cause for it's existence, therefore it is plausibly true to believe that our universe, which is existence, has a cause of existence.
We don't know for 100%, but based on what we do know, we can believe that this is the more plausible reasoning to support.
And most of what you claim to know is demonstrably wrong. Nor is belief a factor at all. You are literally saying "I don't know, therefore god" even though some of the things you don't know, some other people (mostly physicists) do know. All you or Phattonez are doing is invoking an argument from ignorance.
There is not only no evidence of a "first cause", there is no evidence that such a cause is necessary. There absolutely no evidence that such a cause is intelligent, and no evidence that such a cause does not itself require a cause, because infinite regression is automatically a problem that such a cause must overcome. Simply declaring that your cause is immune to infinite regression solves nothing.