Fact and Postulates
Our intellect relies on logic and empirical evidence for making decisions! If we say the sun gives light, it is not a postulate but a fact! There is another element known as trust but it is also outcome of logic to rely on someone from the past experiences or because it is appealing to the intellect through some logic! Sometimes you accept a theory without proper reasoning but there is some reason which may be improper! This is the postulate on which all sciences are based. There is only one exception to it and that is the divine area because that does not lie within the domain of logic and mayic empirical evidence
You can refer to ninth standard (India) book on atomic theory proposed by Bohr, which contains his postulates. Bohr assumed few things to be true and termed them as his postulates, which were proved to be incorrect subsequently! I can list all of them but that is not the topic of the discussion. Accepted truth is the sun has light!
And final answer to proof of God-
Blind person cannot have proof of the light. That does not mean there is no light! Similarly when someone does not have divine vision to see or comprehend God, he or she cannot have a proof of God. The way is to get the vision first. And for that you have to believe in what a person with vision says. And try to get that vision.