I am a believer of God and I will always be until my very last breath. God is the reason why I am existing. He is the air that I breath. He is the giver of life, my healer, my provider and He's the one who provides all m needs to survive. He's the reason why I love to live life each day.
How do you know?
The real question is, do we really know anything?
How many times do people forget their car keys and have to go back into the house or restaurant and get them? They knew they were going to go out to the car and get in and drive away, but it didn't work like that. They had forgotten their keys.
How many times have people gotten into their car - or boarded an airplane - and knew they were going to their destination? But there was an accident. They wound up in the hospital, or morgue.
We know nothing.
How do you know that God doesn't exist to do all those things that Potatohead talked about? You don't know. You simply might believe that God doesn't exist. More than likely, it is only attempted belief in many ways.
Now, here's the point.
God works in the hearts and minds and spirits of those who believe in Him, to cause them to know that He is with them. Potatohead knows, but he knows by faith. Yet it is real knowledge as well as faith. Why? Because God is the One Who is holding the person in faith knowledge, and in knowledge faith.
You don't believe, so you don't have God working in your heart and mind and spirit. Because of this, you don't (and never will) have knowledge that God works... at least not until you believe, and He comes to you.
If you wait long enough, you will die without faith-in-God knowledge. Then at the Judgment, you will find out from God, Himself, that He exists, and would have worked with you if you had allowed Him to. But then it will be too late for you.
How do you know zeus didn't do it instead? How do you know the flying super unicorn didn't do it? How do you know the flying spaghetti monster didn't do it? We don't know when an accident it's going to happen simply because we don't account for all the variables but it's not magic. We have 0 evidence for any god, there is no point in believing in their existence, just acknowledge the fact that some god might exist but certainly not the christian one.