Good question.  These are always difficult questions because I do not know the state of anyones hearts only God alone does.  However I scowered the internet and the typical consensus seems to be this.  (1) Of Mice & Men is not a Christian band.  (2) Vocalist Austin Carlile seems to be a Christian, though maybe not super outspoken.  (3) They would see themselves more as a positive metalcore band.  Many people note the Christian undertones in their lyrics.  This would likely be due to Austin's faith.  Below is a vimeo link to an interview with Austin.