You can prove medical science exists.
When my son was born 3 months pre-mature I put all my "faith" in medical science for his survival. He's doing great now.
Yeah, I tend to agree with averageJOE . . . or, at least, I think I do.
Simply having certain knowledge of something's existence doesn't mean you aren't acting on faith when you place your trust in it.
In other words, to get back around to the OP, just because I am certain of God's existence, that doesn't mean I have faith in God.
I believe the U.S. Congress exists, but I will tell you right now, I no longer have any faith whatsoever in our Congressional Leaders. I don't believe they are looking out for the country's best interest. I believe them all, each and every single one, to be self-serving above everything else. And if there is one or two in Congress who aren't totally self-serving, then I believe they are delusional and misguided to take part in a system that is completely dysfunctional. I know for certain that Congress exists. Yet, I have absolutely no faith in it at all.
Just because a person is certain God exists, doesn't mean that person will place their faith in God, will adhere to God's instructions, will rely on God, will place no other deities before God, will love and respect God, etc.
As a matter of fact, I will tell you right now something else, if it were proven to me beyond any reasonable doubt whatsoever that the God of Jerry Falwell, Pat Baker and George W. Bush really does exists--you know, the God who told the former president to invade the Middle East and to implement tax law that overwhelmingly benefits the wealthy to the detriment of the poor--if that God really does exists, not only would I place no faith in him, I would actively campaign to have him stripped of his divinity.