There's evidence of our creator all around and inside us. Analyze the human heart for a minute, look inside yourself to your deepest desires. It is my belief that our desires were wired for God. We were made in God's image, so He is our definition of 'good' and we naturally need him. So we are faced with a choice, to "drink" from God or earthly things. That's the nature of addictions... we try to dull the desperatly painful need for God with earthly things and depend on them.
It's fascinating to me to see that maybe the strongest evidence of God is in our own design. Check this out too:
http://www.tasteheavennow.net/understanding_yourself/understanding_needs_home.htm
The only way to be truely humble is to just admit and accept that God is God, not us. I don't really like arguing for or against God's existance, because people will hear what they want to hear. If we don't soften our hearts and keep them hardened, and "push the truth away from ourselves" then we won't be able to believe the Truth.
It is not blind optimism, we have a wonderful hope. I think it was Paul who wrote that if these things were NOT true, that we would have wasted everything and would be the most hopeless people in the world. I agree!
God cannot be a subjective thing that we can all interpret the way we want to. He is objective, as is his word and his Son. All other religions, near I can tell, are human's attempts at reaching upward to grasp God with their own humanity and achievments. The message of Christ is the opposite, it's something you couldn't have predicted! God's display of grace, and bloody sacrifice for our sake. To pay for OUR sins and fill in our weaknesses with his strength. But if you set your heart to disbelieve and cling to earthly things to fill your heart, nothing will make you believe. It is an act of the will.
By the way, we ALL sin. None of us can deny it, it wasn't just Adam and Eve. They were the first!