We all know God exists. Why? Because without Him, we couldn't prove anything at all. Do we live our lives as if we cannot know anything? No. So why is God necessary? In order to know anything for certain, you would have to know everything, or have revelation from somebody who does. Who is capable of knowing everything? God. So to know anything, you would have to be God, or know God. For what we do not know could contradict what we claim to know. Without God, at best you have an infinite regression of "knowing" things. You claim to know A because of B, and B because of C, and so forth. But you never land on a foundation for the source of absolute truth.
The source for absolute truth must be uncaused. Why? Because there is no longer an infinite regression. It must transcend the universe. Why? Because to have all knowledge, you would have to be beyond all creation. This source must be a personal being with free will of the mind. Why? Because we as the recipients posses these qualities to comprehend knowledge and truth. All these characteristics conform to the entity of who God is. God is the best explanation for why there is something rather than nothing. For the origin and fine tuning of the universe, objective moral values, universal laws of logic and the intelligibility of nature.
To say I cannot know anything for certain is in itself a claim to knowledge. So it is checkmate that we can only make knowledge claims. Thus there is a God who purposed us in this way to have intelligibility to discover Him through His creation and word. God gave us the ability to know Him or reject Him, but never to know that He does not exist.
Pre-Suppositional Apologetics reveals the fact that we all have within ourselves the knowledge to know God exists. As the Bible says: "For the wrath of God is revealed from heaven against all ungodliness and unrighteousness of men, who suppress the truth in unrighteousness, because what may be known of God is manifest in them, for God has shown it to them. For since the creation of the world His invisible attributes are clearly seen, being understood by the things that are made, even His eternal power and Godhead, so that they are without excuse, because, although they knew God, they did not glorify Him as God, nor were thankful, but became futile in their thoughts, and their foolish hearts were darkened" (Romans 1:18-21).
The proof of God's existence is that without Him, you couldn't prove anything. God is the precondition for the intelligibility of all human experience and reasoning.