I have always preferred a phrase along the lines of "I swear by all that I hold sacred" or "I swear by the gods my people swear by" or simply, "As best I possibly can."

When I was sworn in during a court session I was asked if I swore to "tell the truth, the whole truth, and nothing but the truth?" and I said I would. No god in it, just a perjury charge.

(I wanted to respond, "Up to a point" and when the judge asked "What point is that?" I'd say, "The point where I start lyin'." But our lawyers wouldn't let me and that's probably just as well.)