Q: How do you know God exists? For all you know, God could be made up by humans. Humans WANT to believe that there is something greater than themselves, they WANT to believe there is some big plan for them. The God that you know could be made up just like the other gods that have been made up. Greeks, Romans, Aztecs. They all made up gods. How do you know the God you know was not made up like the rest of them?
A: The full answer to your question requires a discussion of the basic principles of logic, epistemology (the study of how we know what we know), metaphysics (the branch of philosophy that deals with questions about non-physical realities). The short answer to your question is that the God affirmed by natural theology is very much unlike the Titans, Olympians, or other mythological characters from polytheistic religions. My favorite argument for the existence of God is from Bernard J. F. Lonergan: "If the universe is intelligible, God exists." If you read and reply to this comment--whether you agree with it or not--you are demonstrating a commitment to the principles of natural theology by which we recognize that there is a pure Spirit who is the uncaused cause, the first mover, the summit of all perfection, the end toward which all purposeful behavior tends, the intelligence behind all other forms of intelligence; beyond that, we can see that this being is one, simple, infinite, all-powerful, eternal, good, beautiful, and true. Plato and Aristotle recognized the validity of this kind of argument to the existence of God, but the God whom they recognized by the force of reason was utterly unlike the invented God's of the polytheists.
If the universe was unintelligible, you could not say "Humans want to believe that there is something greater than themselves." If there were no God, you could never make claims like this. God is the condition of the possibility of all judgment; every judgment you make implictly affirms that there is a God. (For those keeping score at home, this is an argument drawn from the Jesuit philolsophical tradition of Transcendental Thomism.)
Answer given by Martin X. Moleski