Ethical reason for God:
If a person can deny the existence of God then what would be the merit of placing your hand on a Bible to be sworn into office? What would be the reason for a witness in a court of law? If a person can deny God then they have no remorse to tell a lie. Are there people serving time in prison because of lies from someone who does not believe in God and has no remorse?
If there is no God and they cannot be punished after death then what compels them to keep from telling evil lies? God then also takes on an ethical reason for believing.