What makes music 'Christian'?

This has bothered me for some time now. I think it must of started when the Jonas Brothers started getting more popular. It really bothered me that none of the lyrics on their 2007 cd 'Jonas Brothers' had anything to say about God or their relationship with God. For me the worst part was that all the songs were written by Kevin, Joe and Nick. I couldn't understand why if they were christians why wouldn't they want God to be the center of their music.
Even though I understand it alot better now, then I did, I still bothers me a little bit.
And it really got me thinking about other 'christian' bands. Like FF5. Songs like "Love Addtic" "Replace me" and "Face Down" are amazing lyrics. yet some people have a big problem with them.

So what actaully makes music 'Chrsitian'?
Is it the lyrics? (I'd say no)
The Attitude?
The Band members personal relationship with God?