There are three extra biblical arguments for the existence of God. Those that say that one must turn to the bible for proof for the existence of God are seriously mistaken. Though the bible is a legitimate source for proving the existence of God, there are also three philosophical arguments that make the proof as well.
The ontological argument for the existence of God was developed by Saint Anselm. That than which no greater/perfect can be conceived of is God. Such a perfect being must exist in reality as well as in the mind. Therefore God exists. Secondly, there is the argument of design, known as the teleological argument for the existence of God. The fact of design in nature itself proves that it has a designer--God. The third argument is the cosmological argument. There is cause and effect in the universe so there must be a first cause--an unmoved mover--God.
All three of the arguments prove the existence of God. They are irrefutable!
Obviously scripture proves God's existence. I note the three for those that accuse us of being circular/begging the question by making reference to scripture.