The advertisement is clearly a play on the popular editorial article
He ventured forth to bring light to the world by Gerald Baker. I think it's kinda strange that the CNN article doesn't say anything about it. They almost act like this is the first time anything like this has been said.
McCain is simply poking at the almost messianic light that many paint Obama into. I think it's funny and true in some ways, but I'm shocked that McCain would use such a thing in an ad. I'm ok with some negative ads as long as they are sticking to concrete issues, but this is a case where he's taken it too far. Obama's inexperience is a substance issue, how people treat him like he's Jesus is not a substance issue. Let the media and blogosphere wade in that mud.
I'm sure the only reason he's gone as far as he has already is because it's working. Combined with the Republicans recent stunt today, this has been a fairly good week for McCain.
That being said I think your title is a little misleading. McCain never called Obama the anti-christ.