Many believe the question is unanswerable (yet many others answer it quite easily. In fact, anyone except agnostics).
- There is little agreement on what God is.
- There is no proof to clearly indicate that many of the definitions of God do or do not exist.
- All known indications that there is a God (or gods, for that matter) come from unverifiable sources, such as the Bible, word-of-mouth traditions, or fallible humans.
- The notion of the term 'existence' is hardly explained. The question seems too vague to answer.
Many believe that this answer requires a leap of faith
- Because God is ultimately beyond human comprehension no definition will serve, thus it is hard for people to agree on the nature of God.
- Proof would reduce God to a thing, an element of many like stars, planets or people. Thus the search for "scientific" evidence of God is quixotic at best.
- Note: The above two points imply the rationality of not using rationality to think about big matters (including God). This leaves open the question: What is to replace that rationality? Since we ultimately need to use sth. Or don't we?
- The only indication of God's existence that matters [for some people] is the insight, the intuition that God does indeed exist. This insight [presumably] opens a world of possibilities foreclosed to the mind of each individual human.
- The Bible (The 'the' seems misleading) has long ceased to be word-of-mouth. It is the oldest continually maintained record of "history" in Western Culture. It is not a traditional history in 20th century terms, but its deliberate inclusion of embarrassing and damning criticism of its own people serves as a "witness against interest" which is taken as a high standard in testimony.
So, the conclusion is that "Its you who should decide what belief makes sense to you. Never believe something because he/she told you. Discover yourself and develope something your own"