Merriam-Webster Online Dictionary
1 a (1) : a solemn usually formal calling upon God or a god to witness to the truth of what one says or to witness that one sincerely intends to do what one says
Thanks. I'm lasy. :-)
(the last letter of the alphabet is sticking on my keyboard)
Checking Partridge's Origins, I see it originates from an old Germanic term meaning "a going" -- implying simply "a going-together, an agreement." The term must have gained a more solemn and religious connotation over time.
Still, my feel for the meaning of the word has never been exclusively religious. I've never seen the dictionary like a word bible; it tends to lag behind reality a bit.