I know that God exists, not because of the evidence in the world, but because of the evidence in my life.
Si'er often tells us that the best way to bring across the idea of God is not through physical and historical evidence, but by sharings and testimonies. Yet how many of us truly believe in that? I remember someone once shared this analogy with me: if someone came up to me and said that he got hit by a truck - yet he looks normal, with no hint of injury or an encounter with such a huge object at high velocity - I wouldn't believe him. Yet God is larger than a truck, and if someone says he encountered God - yet he looks the same as always, with no hint of change in any way - who do you expect to believe him?
The biggest testimony to a true and living God is right before you. Your life.