I always have the same song pop into my head when this debate topic comes up. When I was a younger man and used to go to church, there was a song the church sang quite often that had the line "You ask me how I know he lives? He LIVES within my heart"

That line has since become the end all to my debates on the subject. People have a right to believe anything they want to (1st Amendment) and to the extent that God* lives in some people's hearts (and or minds)... He effectively "lives" in our world.