It seems to me that in this ad John McCain falls into the trap of repeating and popularizing his opponents' best lines for the purpose of vaguely trying to punch holes in them.
America is a very religious country, with ninety percent of people saying they believe in God. So a candidate who says that he feels a mission to lead and has been chosen to do so may have a much better change than another candidate who ridicules the religous content of his opponent's mission, without offering any religious content of his own.
John McCain's slogan is, "Barack Obama: He may be "The One", but is he ready to lead?" If American decides that Obama is more ready to lead than McCain, then McCain's McCain may find that it is essentially over.
Religious people take leaps of faith, and "faith is the substance of things hoped for, the evidence of things unseen." If McCain's campaign depends on convincing religious people not to have faith, that makes him a negative naysaying, a tool of doom rather than the man to lift up the nation in troubled times.
Hat Tip to Agent X.