It kinda made me wonder where this expression came from. Actus Dei. Not much help from my fav source of knowledge, Wikipedia. Phrase Finder tells us that it originated from religious texts dating back to the 13thC CE. Not a particularly enlightened period of european history. It apparently began to appear in legal documents in the 19thC.
I would actually prefer the other phase "vis majeur" refering to a "superior force of such a degree that no effective resistance can be made to it". No need to blame God. Our own stupidity, ignorance, incompetence or negligence can also be that superior force.
On another note, I was also wondering what that term meant to atheists, who clearly do not believe in God. Actus Dei? Or actus non-humanus....., or simply, 'whatever it is, don't blame me'.