This can be a very hard topic to describe. I wouldn't say that you have to be clear about what you're singing about in your music in order to be considered a Christian band. Look at underØATH, they're a Christian Screamo band and they're lyrics cans sometimes be extremely hard to interpret, however...
Timothy McTague of underØATH
“Our band has always been this Christian band and we’ve always been open about what we believe in, but there comes a certain point where a lot of the messages in our songs are very ambiguous. And that was so bold and straight up, talking about how we’re all people but without purpose we can feel lost."
Hence telling you this band is Christian, without any doubt. So I say it's up to the bad to decide if they are Christian or not, not up to the fans.