We use it because it seems to make sense, and if it did not we would not use it. The core axioms were selected for their utility, not because they have some transcendent philosophical significance. As long as you presume otherwise, you will be wandering blindingly. As I mentioned previously, there is not even perfect agreement among mathematicians as to which axioms we should and should not be using.
God is like that. Sometimes called the "uncaused cause", He provides a transcendental unifying origin for all of the axioms/trasncendentals that we take for granted.
So you invent another axiom ("God") to create a relationship between the other axioms. What you have just stated not only is semantically null in that you are shuffling axiom deck chairs to no substantive consequence (it creates no new derivations), your particular deck chair arrangement violates Occam's Razor. Per your argument above (using a unifying God axiom), the mathematical proof of Occam's Razor still holds and so your argument can be tidily invalidated from your own premises. Any hypothesis that fails the test of Occam's Razor is ipso facto irrational as a belief.
Get it now?
It is fundamentally opinion-based. You have obviously expended enormous energy to assuage your fear of consequences in the next life. I respect that - your emotional involvement is quite evident. May I point out the following: Your own reductionist reasoning is circular at the top as well - in (negatively) not assuming God's existence you implicitly, POSITIVELY, assume His non-existence. Your use of Ockham's Razor fails to account for built-in prejudice. There is no such thing as neutrality - yours is pretended and the product of the cumulative experiences that comprise your noetic make-up.
Your real motives, from what I can see, are emotional in nature. Which is quite understandable...