That's a good question.
In my definition, a Christian metal band would be the following:
1) A band who self identifies as Christian, and reflects their beliefs in their lyrics.
2) A band who participates in the Christian music scene (in other words, plays with other Christian bands, appears in Christian websites and magazines, etc.)
3) Plays music which falls into a traditional sub-genre of metal (i.e. power metal, doom metal, death metal, black metal, thrash metal, etc.)
and 4) Self-identifies as a metal band.
The most important thing, I would argue, is that the band or fan identifies personally with the "Christian metal" community, if that makes sense.
This research will have two phases- right now I am conducting interviews in Chicago as pilot research for a class I am in. After that, I will expand the research to include the whole U.S. as I spin this into a Ph.D. dissertation project.
Feel free to PM me or e-mail me if you would like to talk to me.
Have a good weekend!