Quote Originally Posted by §håd0w
Of course you can... Christian Rock.

If you can't give a genre based on lyrics, then most metal would be classified poorly. Death metal generally has recurring themes of dying in their music, as with Christian Rock is all religous.

And going with Emo... If the band didn't have whiney lyrics, you couldn't call them emo. So yes you can classify a genre of music based on lyrics.
Death metal is defined by its musical characteristics. Lyrics are often about death, but not always. And no, Christian rock is IMO not a valid genre. There are genres defined by the lyrics, but not within metal. There may be a common lyrical theme in certain genres, but it is not relevant to which genre the music in question belongs to.