By "God" I just mean some vague concept of a Supreme Being. When I was in high school, I considered adopting agnosticism, since I wasn't sure that God existed, but I also felt that atheism went too far. It was Thomas Aquinas's Fifth Way, which can really be traced back to Plato and Aristotle, among others, that convinced me that a Cosmic Designer (God) exists.
The Bible itself makes it clear that God's existence is obvious. "The heavens declare the glory of God; the firmament proclaims the works of his hands." (Psalm 19:2.) "The fool says in his heart, 'There is no God." (Psalm 14:1.) And finally, "For since the creation of the world His invisible attributes, His eternal power and divine nature, have been clearly seen, being understood through what has been made, so that they are without excuse." (Romans 1:20.)
Why think what the Bible says is true? I've repeated this version of the Fifth Way many many times, but it never ceases to amaze me how obviously true it is. If you're offended by some perceived hubris on my part, I don't know what to tell you. I find God's existence to be an obvious fact.
1. The order and regularity of the forces of nature are either the result of chance, necessity, or design. (Premise)
2. Whatever exhibits order and regularity is not the result of chance. (Premise)
3. Hence, the forces of nature are either the result of necessity or design. (From 1 and 2)
4. They are not the result of necessity. (Premise)
5. Therefore, the forces of nature are the result of design. (From 3 and 4)
As the Cosmic Designer of the forces of nature, this being must transcend nature/the universe, which is the sum total of all physical space, time, matter, and energy. Therefore, the Cosmic Designer must be timeless, changeless (for time is a measurement of change), immaterial, and enormously powerful and intelligent.
Given its timelessness, the Cosmic Designer must also be Pure Actuality, which necessitates its immutability, eternality, indestructibility, unicity, omnipotence, omniscience, and perfect goodness. These latter attributes require some more deduction, but the former attributes suffice to demonstrate the existence of God, or at least something very much like God.
When I talk to people about this argument, even if they had previously thought belief in God was purely a matter of faith, they almost unanimously agree that belief in God is unavoidable. They realize that atheism is simply untenable.
What I find interesting is that I don't find that Fifth Way to be the best argument for God's existence (I think the First Way is), but I do think the Fifth Way is the most obviously true argument for God's existence. Even the skeptic David Hume could not deny the obviousness of design exhibited throughout the cosmos.