Is it permissible to accept and rely upon the oath of a Christian friend who says, "By God," even though he is not a Muslim, and is his oath considered to be by other than God if he intends Christ, and is his oath to be taken into consideration outside of a court of law?

There is no harm in believing a disbeliever in worldly transactions, but not in matters of religion. Jurists have held that the testimony of a disbeliever regarding his actions is acceptable, and his oath is accepted if he swears by Allah. His oath is considered like the oath of a Muslim in matters of rights. As long as he has sworn by Allah, he intended the Creator, and that is sufficient. It is advised not to dwell on whispers of disbelief.