Christianity just like all faiths play roles in metal lyrics. They might not always say praise God and love him etc but they might take stories from the bible etc and write about them because it is interesting. So I find nothing wrong with Christianity or any faith in metal. What you believe is what you believe, and if you are musician those beliefs should play a big part in your music. I don't see why Christianity in metal should bother any metal fan, if you like the music and the message then listen to it, if you don't like the music and the message don't listen. I don't listen to Christian bands much, I started listening to Stryper recently but that's the only one. I have heard a few Christian metal songs from different bands, and I am fine with it. They don't seem like they are forcing their beliefs on the listener, they are not talking about hate and violence towards another like a lot of ignorant Christians do.
I use to be bothered by Christianity and anyone who talked about how they love God and Jesus, but that doesn't bother me anymore. Love and peace is the true message of Christianity, that is the message of all faiths. Im sure there was a good message in Christianity and other faiths, there still is a good message, the true message is peace and love, anything that deals with bringing positivity to yourself and others, but that message has been buried, corrupted and twisted by people in power to fit their own hate filled agenda of violence and war. Religion was created to indoctrinate and divide the masses. Religion tries to destroy any idea that tells us we should be one with each other and the universe, it tries to destroy anything that says we should not preach hate and violence but instead live in love and peace.