only that it has its "earliest moments."

Fine, lets go with that. He made that.

we first have to agree that we're talking about precisely the same thing.

I can tell you a spaghetti cook is someone who makes spaghetti. You don't need to know what kind of ladle was used. You don't need to know how long the spaghetti was cooked for or if he put salt into the water. We can disagree on all of those things and still agree on the definition of a spaghetti cook. Similarly, we don't have to agree on every single detail about how the universe was created, whether or not time and space are linked, exactly how time worked during that time. However it worked, whatever the answer is to that, god is defined as the dude who made it. I'm trying to express through example why you don't need to know every detail about how something is created in order to define a creator of that thing. We don't even know if the chef was involved in making the pasta. Is the sauce from a can? It doesn't matter. Its a catch all. In either case, its the guy who made the pasta that is in the plate in front of you, ready to eat. Maybe he made the pasta itself. Maybe he didn't. Either case will fit the definition.