Existential writers such as Søren Kierkegaard claimed that proof of God cannot be the outcome of a logical argument, such that God’s existence can never be a public or objective truth. Belief in God, consequently, must always be a private matter, entirely subjective and a function of the individual accepting such truths for themselves as a matter of faith. Hence attempting to prove the existence of a God via such means as the Argument from Design would not fly in Mr. Kierkegaard’s neighborhood.
However, the way I see it is that the way most people accept the existence of a God is along the lines of believing seemingly something far less profound, e.g., believing Paris is in France; although you may have never been there, you accept this to be a true fact about the world. It fits in with what you have been told about Paris from the time you heard it first mentioned, from what you heard at school or from what you have read about it. As such, the truth of such a belief and many similar ones like it is a function of coherence with other beliefs that seem to support it, giving you no reason to examine it critically or ever doubt it for that matter.
I’m willing to concede however that – when people say they believe in God – they might be expressing more than just something that they have always accepted as true, such as a belief of the “Paris is in France” variety. What may be referred to as “spiritual beliefs” are the results of having a sense or an awareness that one is part of something larger and more profound than oneself while being unable to cite the specific reason for believing this to be a true belief about themselves and the world.
Beliefs of that nature may have some intrinsic credibility based on the phenomenological nature of our everyday experiences, when one is led to expect a greater context for them beyond the immediacy of the present moment and whatever else one might bring to bear on them. It is within this expectation or awareness that one might ascribe to the possibility of a deity existing, especially when one is told from day one that there is such a thing as an all-powerful being named God, the culmination of religious brainwashing at the hands of some authoritarian religious institution.
Given this line of reasoning, you could say that the belief in God merely fills the void in one’s belief system that resulted from sensing the larger whole of one’s existence without being able to articulate exactly what that is. That the myth of God could be an answer in all its stupefying simplicity as to how to account for the larger world at the experiential or phenomenological level says a lot about the spectacular gullibility of our species. As well, it goes a long way towards explaining the willful manipulation of people by religious institutions over the centuries, to no other end than being able to control them, use them or – if they threatened their vacuous authority – by killing them.