How is believing in God good for your spiritual development?

Its very simple.

To believe that everything is created and sustained by One source and Power, makes me feel content, humbled.

I remember that were all equal in the face of such Power.

Were all equal and we ALL have equal chances reaching happiness :)

Because you are using capitol G god I assume you are talking about the Christian god?

Believing it it was good for my spiritual development because when God never did all the things I was taught he would, I began searching for the real power in the Universe.

It is not good.

"To believe that everything is created and sustained by One source and Power, makes me feel content, humbled. "

It discredits the intelligence, power and spirit of the human race to adapt to and overcome obstacles, and makes you unable to find your own answers. "If it gets too hard, attribute it to God - you won't have to think about it".

We are all equal, but for such physically weak and unprotected creatures, we have come a long way, and we should give ourselves credit - not God.

Be aware that there is more than meets the eye to your regular human being. But don't search for answers in the bible - you will not find one.