That matter, energy and especially life is eternal, takes far more faith than "God did it all."
AFAIK, no-one seriously maintains that matter or life can be eternal - the conditions thought to hold at the big bang preclude that. The same applies to energy in some hypotheses (the net energy of the universe is zero, which suggests a universe can be generated from a zero-energy metric, and other aspects of known physics support the idea).
How can there be various theoretical models for an eternal cosmos?
Because we have incomplete data. The models are hypotheses and depend on differing assumptions. The principle is the same as if you see a tiny light moving across the night sky - you could formulate a number of hypotheses to explain what it was and/or where it came from - it might be a satellite, or an aircraft, or a meteor, or a balloon, or a drone, or an insect, etc. You could then try to support or eliminate each hypothesis by a closer examination of the information you have (how fast is it? is it steady or blinking?, does it make a noise?, etc.), or trying to obtain additional information.
Either it was or it wasn't.
One would like to think so...
Although there are some ideas that involve the universe being closed in time, so time has a beginning and an end, and the universe just
is (i.e. completely independent and self-contained, with no before or after, and nothing 'outside').
It's tempting to think that if you can't visualise the ideas or wrap your head around them, they can't be real, but quantum mechanics has shown us that isn't always the case; sometimes we just have to rely on the mathematics.