Acquinas had five ways to prove that God existed (summarized)
1. Motion. Since things like planets are moving, something must've put them into motion.
2. The Cause of Existence. All things are cause by other things, nothing can cause itself to be, there cannot be an endless line of obejects causing others to exist, therefore there must be an uncaused first cause (God)
3. Contingent and Necessary Objects. There are two types of beings in the universe. Contingent beings can't exist without a necessary being causing its existence. So contingent beings are caused, not every object can be contingent, therefore there must be a necessary being to bring about these contingent beings.
4. Degrees and Perfection. Humans percieve different degrees of beauty, beauty is a quality. Therefore for any quality there must be a perfect standard.
5. Intelligent Design. The unviverse works in such a way that one can conclude it had an intelligent designer. All physical laws and the order of nature and the universe were set in motion by God.
(Source: http://members.aol.com/plweiss1/veritas.htm)
So, what do you think?