Remember when you were a kid, and you believed in the tooth fairy? This little thing would come in the middle of the night, keep your tooth, and leave you money (but what did she do with all those teeth?!). You never saw her. You just believed simply because there were signs of her existence. Sure, it turned out to be your parents, but for years, you believed! When it comes to God, we also have to believe without seeing.
We know He exists because of the signs. This beautiful world did not create itself. Someone (not something) set it all in motion. Someone made you… in His own image. When you actually take the time to look at the world around you (and the people in it), it becomes impossible to ignore the signs. Like the tooth fairy, you can’t see Him. You have to rely on those signs. But unlike the tooth fairy, God is real.