I'm not sure God is addressee here. I think "God grant" is analogous to "God forbid" - an expression. But if you think that a comma should be placed, it's OK, this wouldn't corrupt the general meaning...
Via, actually this is not a formal "prayer", it's one of Reinhold Niebuhr's famous quotes.
I'm sure this should be the correct punctuation.
I'll set a poll, but of course, it will soon be accepted and the points in your pocket