On the topic of deity, one must start with what it known, and from there move to what is most likely, given available evidence.
To begin with, there are three possibilities, one and only one of which is true:
1. There is no god. (The atheist belief)
2. There is exactly one god. (The monotheist belief)
3. There is more than one god. (The polytheist belief)
It is notable that the last two are not as straightforward as many think. For instance. God vs Allah. Is one right and the other wrong/false? Or are both referring to the same one god? Or Are they both real, and separate? Dueling it out in some great astral chess game in which we are all pawns? The latter scenario takes what are generally thought to be beliefs systems of type 2 and defines them as type three.
Likewise many type 3 belief systems could also in fact be type 2. Some view Hinduism as having many - even millions of gods and goddesses, others say there is only one deity, that the "many" are merely different manifestations of the same "one". For other multi-deity belief systems, such as Greek and Norse mythology for instance, It is possible that these system were originally a single god system, but over time, the stories began to change, and the one being was re-imagined as multiple beings, perhaps to make it easier to rationalize what the believers saw as behavioral discrepancies in said deity.
Similarly one could argue that monotheistic systems performed the same process in reverse. To determine which way such a process went, one would need clear timelines for the various stories so that one could which were closer to the source stories, and which were more altered re-tellings.
In the absence of concrete evidence, we must assume that all three of the above possibilities are equally likely.
Some would claim that there is concrete evidence - Dinosaur bones for instance, which prove that the world wasn't made in 7 days as the Old testament claims, this portion of the "Old testament" is a key part of Christian, Jewish, and Islamic faith. But all this proves is that the biblical claim that the earth was made in seven days is either incorrect, or misunderstood. It is possible the biblical telling is meant as more poetry than prose. It is also possible that the story was altered over the years of telling and retelling (what we have was either written by Moses, or by someone who heard it from Moses, making it at least one re-telling from Moses, and possibly more. But who did Moses hear it from? It could have been re-told and altered for several generations by the time it got to him).
Another possibility is that the writer or original teller (God?) may not have been talking about earth days (What is a day to someone who isn't from our Earth?) According to the Genesis telling of the story, The Sun, Moon and Stars don't come into play until the 4th day, suggesting that it wasn't earth day that was being used as the measure of time.
"But if the bible is imperfect or incorrect, then there must not be a God" right? Again, this is not proof of the nonexistence of God, rather it is proof that some theories about God are incorrect. The fact the the bible, prophets, and the world are imperfect leaves several possibilities in which God exists:
1. God is imperfect.
2. God does not micromanage.
3. Imperfection is part of the design.
4. We have an incorrect understanding of the word "perfection" where deity is concerned.
(typically about this time someone goes on a rant about religious people and their double-speak. This isn't doublespeak. It is merely the nature of language. Language is imprecise and fluid. Meaning changes over time, and from one person to another. The goal is to reach beyond the syllables to the personal meaning. Real communication isn't speaking at each other, it is understanding each other.)
So, an objective review of the facts reveals that the facts neither categorically prove, nor categorically disprove the existence of God. What then are we left with? Speculation. Theory. Belief. Hope. Faith. And "personal science"