"The heavens declare the glory of God; and the firmament sheweth his handywork." [Psalm 19:1]
On many occasions, when asked about their belief in God, I've heard people respond along the lines of "I just can't believe in something for which there is no evidence." Yet we as Christians look around and see evidence of God everywhere: the design of creation, the complexity of life, morality, logic, love, etc. If some simply cannot see God in these things, does that mean there's a problem with the evidence?
No. Rather, there is a problem with their world view. Your world view is made up of your most basic beliefs and is what tells you how to interpret evidence. The atheist's world view assumes that God cannot exist, and so he interprets evidence with this presupposition. When he looks at the design of creation, he cannot see the evidence of God that is right before him because his world view has filtered it out. It's like wearing glasses with a big dark spot in the middle of the lenses. If you look directly at an object, say a book, you won't see it; not because it isn't there, but because what you're using to look at it prevents you from seeing it.
When debating the existence of God with someone, you must first examine their world view and try to correct that. Otherwise, no amount of evidence, no matter how powerful, will convince them of the truth that is before them.
(more information on this topic can be found in an excellent 4-part video found here.)