Is There a God?
No one can prove to you the existence of God, as in a mathematical proof. But you still will be able to be certain of His existence:
There are many good, rational arguments. We will make them in a moment.
But if you do not want to believe in God, you never will. There can be no scientific proof, simply because God is not physical or material, He is spiritual and infinite. Science is the observation of material phenomena in this universe, and then applying our reason and logic to understand and control them. By definition, God cannot exist as part of this universe, cannot be composed of matter, and cannot even exist in time.
But even things in this world cannot really be proven with 100% certainty. Entire books have been written about how the U.S. space program has been faked by the government. All you have to do is set the standards of proof high enough, and absolutely nothing can be proven. Proof is a tricky subject. As the old Greeks like Euclid discovered, all proofs have to rely on at least several assumptions (which they called postulates) which cannot themselves be proven. So a person who demands hard proofs is doomed to failure.
It is a doctrine of the Church that faith does not come to us through reason alone, but by the grace of God. If a person opposes even the possiblity of God's existence, then any arguments or evidence can be rationalized away.
Since it is impossible to "prove" with absolute certainty, any amount of belief in God has to come from your own spirit, from within your own heart, because it is the spirit of God that we are trying to find. The paradox is, you will only find this faith if God gives you the grace to find it.
A person of faith already knows why they believe. It is not for any selfish reason, or because they want to be "saved." It is because they know they are a true child of God, and that we naturally seek Him and know Him as only a child can know their true parents. It is a completion of our lonely existence. It is a feeling of coming home. And it is a certainty that goes far beyond all efforts at "proof." Our faith is a pure gift from God. It is the result of our having accepted the gift of His grace.