I believe in Truth, with a capital "T." Universal, undeniable, eternal Truth. Augustine of Carthage believed in Truth, too, and he proved its existence with math. Not the approach I would have taken, but it served his purpose. Basically, he said that in an equation such as 7+3=10, we all know the answer to be true, it has always been true, and it always will be true. We can not change it, no matter how sharp our faculty for reason or how ardently we claim not to believe it.
Truth does not require belief, or anything beyond its own existence, to be Truth; its existence is intrinsic to its nature and in it are contained all knowledge, reason and sense of morality. There must be a being which embodies universal, undeniable and eternal Truth, since existence, or being, is part of Truth's nature, and this being is no other but God. When one discovers Truth, one discovers God.
C. S. Lewis was an atheist for decades, arguing that if a good and all-powerful God existed, He would not allow evil to exist. But then he saw the hole in the argument: if there is no God who is Truth, how are we able to recognize evil, the lie, to discern right from wrong? There must be a Truth, a moral absolute, a God who created us and instilled us with this knowledge.
Without Truth, the universe, and everything in it, would not exist. There would be no rationality or order or reason or morality. There would be nothing, what the Greeks called Chaos. Since this Truth is so vital to our very existence, it is natural for us to want to know it, to discover it and understand it, and since the dawn of time humans have strived to do just that.
The Greeks, like Augustine, employed mathematics, pure reason. The universality of numbers suggests the existence of something higher than human reason, and the Greeks developed philosophy for the purpose of finding out what that was, of discovering Truth. Philosophy, which means "love of wisdom," was born out of reason, and gaining wisdom from the search for Truth was thought to be the highest aspiration.
The ancient Hebrews approached the search differently. They believed that only by following the Law would they attain Truth. By adhering to the Law and believing in its Maker (the source of Truth) they fulfilled their purpose.
The Greeks were people of reason; the Jews, of faith. In early Christianity these two approaches to Truth caused misunderstandings, which lead to tension and disagreement between Jews and Gentiles, "for Jews demand signs and Greeks desire wisdom" (1st Corinthians 1:22).
Traditionally, Christianity has upheld the Jewish side of the debate, since, after all, Jesus was a Jew, and tended to reject reason as incompatible with the faith required for salvation. But Clement of Alexandria, a second century Christian writer, claimed that Greek philosophy, like Jewish Law, was a "schoolmaster," or preparation for the Truth of Christ: "for philosophy itself did once justify the Greeks." And indeed, the Greek philosophers did reason their way to monotheistic belief centuries before Judaism ever spread much beyond the Jordan River Valley, when all other cultures in the world, with the exception of Egypt for a few years, had always been polytheistic. Their reason lead them to God, though they did not know it.
While the Jews awaited the fulfillment of the Law, the key to their faith, the Greeks sought and found the answer to their philosophy, but didn't recognize it until Jesus came. He came to fulfill the Law and justify by faith, but also to be the Answer to all questions asked by reason. Jesus is the Messiah of the Jews and the Christ of the Greeks. He is the author of both faith and reason. He is Truth.