"First, when the universe began there was no God"

I don't see how this follows or doesn't follow from physics. Certainly, according to physics, God is not necessary. But to most believers God is not something that cares about physics, because God exists outside physics. Physics can only comment on what is governed by physics. Telling people they are ignorant does not refute this basic objection.

I'll put in another way. Perhaps we can say everything we want about the closed (let's just say 10-dimensional) manifold on which we live. We can pinpoint exactly where the negative signature of the metric degenerates, we can solve Cauchy problem for all time, or whatever. How does that tell us any information about a larger ambient manifold, or any disconnected components of the manifold? After all, Nash's Embedding Theorem holds for all signatures.

TL;DR: It's a logic truth, if you accept the Big Bang theory, then God couldn't exist before the beginning of the universe.

I think you didn't understand the premise here.

The premise is: "The Universe started in the Big Bang.", now, following from that premise it's impossible to have a God before the Big Bang.

You can contest the Big Bang theory and with it find a loophole for the existence of God, but the OP was finding it "amusing" and nonsensical what actually follows directly from accepting the Big Bang theory: God couldn't exist before the beginning of the Universe.

This assumes that the Universe contains God. Most understandings of a divine creator place him outside the bounds of the physical universe, and therefore immune to your argument. Unless you're referring to a specific idea of "God" that does include him/her/it as a part of the universe.

If the God is outside the physical bounds of the Universe then he cannot interact with the Universe. From the moment the God can exert some change or even just observe parts of the universe then he must be within the physical bounds of the Universe.

Again, the issue here lies in your understanding of the Big Bang theory.

Let me present an alternative scenario, if a video game developer creates a virtual world, is he required to exist within the virtual world to exert any change or "even just observe parts" of the virtual world?

Ok, but this is just playing with the word "before".

>It'a quite clear after you understand semi-advanced physics that nothing could exist before the universe started.

The implicit argument here is limited to the type of things that make up the universe we can see and observe. It would be most accurate to say that the things that make up the universe could not have existed before the universe started. Philosophically speaking we can't preclude the existence of things that could be of a different type, or exist outside our own universe. It's the same problem explored in Plato's cave.

Sorry, but you just don't understand physics and cosmology. It's not about the "type of things that make up the universe we can see and observe" it's about ANYTHING that makes up for the universe, including dimensions for instance.

This is exactly the issue that I was pointing. People discussing these themes don't have enough knowledge to discuss them and they enter in easy - but totally incorrect - getaways like the one you just did. And when trying to explain them the science behind it, they realize it jeopardizes their faith and refuse to learn it.

Your argument assumes a level of understanding in cosmology and physics that does not currently exist. The standard model, as it stands is incomplete and does not predict all observable phenomena, it is intellectually dishonest to imply that we have fundamentally answered these questions empirically.

I'm sorry, but it's your level of understanding in cosmology and physics that is the problem here.

1 - I'm not claiming that the standard model is the ultimate mode. I'm just claiming that following the assumption of the Big Bang which is part of the standard model, then the existence of a God before the creation of the universe is a de facto impossibility.

2 - Something that may or may not exist outside our universe, can't have any influence or even be able to observe our universe. This, is not a theory, it's the definition of Universe. Thereby, the most your could say from the Plato's cave allegory, was that a God created the cave, but that the rest of the world where that God was must have existed before he himself came to exit.

The key difference in our viewpoint here is that you take the standard model as dogma, and I do not. It is incomplete, subject to amendment as new facts are discovered as any good scientific model should be The logical progressions of our arguments diverge from there, understandable so.

>>but I've never met an astrophysicist that was religious

You should visit the IAS in Princeton sometime.

I believe they exist, I've just never met one personally to discuss these things.