All educated Catholics must have come to the conclusion that there is a God. The way they ultimately came to their conclusion may be different; some people may believe upon faith alone, but others might have based their findings upon basic logic. Without a God there are so many basic questions about life left completely unanswered, such as, how did the Universe begin? There are many different proofs to the existence of God, and the type of proof used depends on the person being spoken to. .
If I were trying to convert a scientific-minded person then I would use the proof of origin from design. There are two premises to this proof. The major premise states that is there is a design there must be a designer. Our computer did not just luckily fall into place in order to be useable it must have had a designer that put the parts together. Humans are the same way. Without God there is no designer and humans simply were made by luck. The minor premise says that the universe has a design, or an intelligent mind behind it. Our universe has seemingly been designed specifically for human life. If the sun wasn't exactly where it was then we would not be able to live on Earth and we already know that the other planets are no suitable for humans. Things are perfect for our lives and that is due to the supreme designer, God. The odds of these things happening by chance are greater than, " a Shakespeare play emanating from the random key strokes of ten chimpanzees." This proof is good for scientific minded people because they know just how improbable it is to make the universe so perfectly with the exact conditions needed, so they could not deny this proof.
When trying to prove the existence of God to a person who is unsure what to believe is the existence of miracles. Miracles are things that happen that neither nature nor human power can cause. Only a supernatural power can cause them.