1. The traditional Islamic/Judaeo/Christian God consists of three main traits, omnipotence, omnibenevolent, and omniscience.
2. I haven't read much on "being" or "god", but I am certain some philosophers out there have argued of the meaning "be" and "god". How could God/ a god be? Can existence exist? Descartes thought of such, you may have heard of his rather infamous Dream Argument, and "I think, therefore I am."
3. The sheer numbers of probability in the creation of the solar system, sun, position of the earth, composition of the earth and so on may have resulted in the belief that our existence was not random, when in fact it may just be so. Maybe humans are just too grateful about life to give credit to probability and numbers.
4. The Bible itself says that it was written by men inspired by God. And I agree with people when they say that self reference is not a good justification. WIP.
5. The possibility is arbitrary. To say the least. You can look at this as how Pascal did once. Search up Pascal's Wager. "Pascal argues that a rational person should live as though God exists and seek to believe in God. If God does not actually exist, such a person will have only a finite loss (some pleasures, luxury, etc.), whereas they stand to receive infinite gains (as represented by eternity in Heaven) and avoid infinite losses (eternity in Hell).[2]"
https://en.wikipedia.org/wiki/Pascal's_Wager
6.