Starting with our own creation in our mothers’ wombs from a tiny cell, to all kinds of other complexity and harmony that we see around us after we are born, the signs of a source of intelligence are overwhelming if we manage to look carefully. An unbiased logic can bring us to the point that there should be something behind the curtain somehow, but at the same time since it is not possible to comprehend how such being can exist, our minds get confused about how to proceed.
Quran helps us at this point with identifying our Creator using four words: the First, the Last, the Evident, the Hidden. Its existence is evident based on Its endless signs, but It is physically hidden from our dimension, and even though our minds will fail to understand how, some being must have always existed, and must be able to always exist, in order for anything to exist anytime, anywhere. For a thinking human, and again without any bias, these logical conclusions are quite clear.
Even though it is not straightforward to reach sureness about it, it is at least obviously the more likely explanation than the alternative where things are popping into existence from nothingness, or that it is unconscious matter that has always existed, and that the incredible complexity at every single level of existence from sub-atomic particles to life and to the whole universe is organizing itself without any awareness.
So a human can either choose to follow reason honestly and try to understand further, or ignore all of this by choosing to go against reason, and cover this realization and bury it deep in memory acting like it doesn’t exist anymore. The concept of disbelief in Quran is an active rejection of this intellectual observation rather than an absence of “believing”, while the concept of belief is not an action of blindly accepting some religious creed, but following reason and seeking knowledge in awe.