Any spiritual belief will do.
And I thought that the deity mentioned in the oath was once meant to be the Christian god and not some pagan god or Mother Earth or anything like that.
A common misconception. If you read the very first Boy Scout Handbook you can read a section where they accept Islam (quaintly called Mohammedeism at the time) and Native American beliefs as being part of spiritual beliefs acceptable to the BSA.
I'm sorry but this really looks as if being superstitious is a requirement of being a scout. Or am I wrong when I assume that the average Christian considers Paganism/Wicca, Mother Earth worship and other New Age stuff to be mere superstition? As far as I can tell, some even think Islam is just superstition (but hey, they can join the BSA).
I would not know what the average American considers Paganism, etc., as.