Its not undercut at all by not winning the popular vote. 81.6% of the electorate identified as Christian in 2001 and 78% by 2012. Thats a clear majority of the electorate.
Being Christian probably neither means that your favorite book is the bible nor that you would rather vote for somebody whose favorite book is the bible.
No one is saying the Bible is the favorite book of Christians nor that its enough to sway votes. The claim was saying his favorite book was the Bible connected him in some form to a majority of the electorate which it did.