How is it proof of God's existence?
There is a ratio that exists in all living things that makes sense...1.618xxxx etc. If it were any difference we would not be here...the fact we are is not proof of God but proof that for us to exist that ratio needs to be...nothing more.
The laws of mathematics is universal and encompasses all things. But doesn't mean there is a God...just means what is is and that is the reason why it exists.
All order in the universe is evidence of, is order in the universe, it only requires set physical properties that we refer to as laws, in order to function in an ordered way.
What believers need to find is evidence for the existence of a god, that cannot be interpreted in any way except by the existence of a god, then they need evidence that it's their god, not any of the many other gods from other faiths.
I'm afraid "I believe" doesn't cut the mustard when it comes to talking about evidence.
The golden ratio doesn't justify the existence of God. Two values are in the golden ratio if their ratio is equal to the sum of the values divided by the larger quantity. That doesn't justify God. Sure this shows up in nature, but it's been proposed that the growth pattern in some plants that produce Fibonacci sequences are the most effective growth patterns. The Fibonacci sequence is a very natural progression of numbers and their sums. It also does not show up everywhere so it could be coincidence. We don't know the exact reason why it shows up so often in nature, however, proposing that God is the reason is just another "god of the gaps" argument that is an illogical position to take. It's also been demonstrated to be wrong many times throughout history. Where people throw god in to explain phenomena they can't, science eventually does explain those gaps in our understanding.
Saying that "god did it" is to me so dull-minded and boring and just shows that one lacks a curious and inquisitive mind. Never stop asking questions. Jumping to conclusions leads to misconceptions and missed chances at discovery and learning.