Why did St. Thomas think God was real?

Roman Catholic AnswerWhen Our Blessed Lord appeared to the apostles in the locked room after rising from the Dead, Thomas was not present; later, when He came again, He said to Thomas (from the Douay-Rheims New Testament, St. John20:27-29:

Then he said to Thomas, "Bring here thy finger, and put it into my side; and be not unbelieving, but believing." Thomas answered and said to him, "My Lord and my God!" Jesus said to him, "Because thou has seen me, thou hast believed. Blessed are they who have not seen, and yet have believed.