This is the first supposition, statement, assumption, of all the monotheistic creeds whether they put it first or not. Usually it is implied, sometimes it is not. Maimonides stated it out and you can get a good beginner's overview of his principles at MyJewishLearning.com here.
Notice that there's no reasoning to back him up there. I am unable to thumb through a translation of his Guide for the Perplexed to find his reasoning and for the love of G-d I hope he was blessed enough to keep his mouth shut and pen in the ink well on that. Why you ask?
As Douglas Adams pointed out regarding the Babelfish in The Hitchhiker's Guide to the Galaxy:
Now it is such a bizarrely improbably coincidence that anything so mindbogglingly useful could have evolved by chance that some thinkers have chosen to see it as a final and clinching proof of the non-existence of God.Really now, do you want to be the one who negated G-d? In a Zen-mystical sort of way, chasing G-d is the surest way to never find Him and chasing His proof is the surest way to hide him from yourself.
The argument goes something like this: "I refuse to prove that I exist," says God, "for proof denies faith, and without faith I am nothing."
"But," says Man, "the Babel fish is a dead giveaway isn't it? It could not have evolved by chance. It proves you exist, and so therefore, by your own arguments, you don't. QED"
"Oh dear," says God, "I hadn't thought of that," and promptly vanishes in a puff of logic.
To digress a slight bit and explain how that can be meaningfully... it is generally true in most endeavors that the more you obsess over a single thing the more likely you are to miss it. The search for a misplaced favorite pen, when not undertaken for the sake of actually finding the pen, but possessing the pen, will make you miss important non-pen-related facts, like having been wearing a certain jacket when last you used it. The pen, the pen, I MUST HAVE IT!!!
Neglect of this fundamental aspect of human psychology is not coincidentally the same reason for people not being able to grasp that though the journey and not the destination is your whole point, the journey is given form by its destination in the first place. If you obsess over the path of your journey to G-d, you won't ever get there either, but probably will bump into a wall, fall down the stairs, and trip over your kid's shoes while you keep your head down.
To make it easier for the visually fixated, you can never walk to the horizon, but that doesn't mean it isn't a good direction to be walking and the path won't be the same if the destination is Burger King. You'll actually get there and not have any path left to enjoy. So you're left to choose a path whose destination is not only unachievable but nonetheless a good one to walk towards because it creates a good path. Which involves Proto-Principle #1: Good and bad are real and definable.
I like to remember the line from Farscape by Harvey Scorpius to John Crichton which you may watch right here:
"Well I think that like religion it's an individual choice. Either you believe and therefore bunnies are unnecessary or you don't, in which case, chocolate!?"That works pretty well. Except with my usual taste for uniting opposites to balance and cancel at the same time giving neutrality and serenity... I'll take the chocolate AND G-d.
Summary: You choose a place to go (wherever you believe G-d is) that defines a path between it and where you are now (where G-d already is but you don't believe it) and since that path is the one you should be on, you are now at your true destination and you didn't even have to do more than make a choice. Wait, dude, what? Oh, hi G-d.