Atheists, tell me, a Roman Catholic: why should I become an atheist?

[Ignorant]

That's a very important point because it renders God an unnecessary additional assumption on that level of the discussion.

I agree that it is a very important point, but it only seems to render a particular set of god conceptions as unnecessary additional assumptions. There is at least one concept of 'god' that would still be a "necessary additional assumption", i.e. god as metaphysically necessary "being". According to that metaphysic, nothing could "be" at all, even a universe understood as an infinite series of change ("beyond" the big bang even), unless "being, itself" is. Of course, you might find this metaphysic inadequate, just throwing it out there as a possible view of reality that can not only account for both an eternal universe and god, but logically concludes the relationship as necessary rather than presuming it is so.