Petra are far and away the most popular--and important--Christian rock band of all time. They pioneered the idea of using rock forms (particularly anthemic arena rock) as a means to spread the gospel of Christ, and in the process they sold millions of records and gave rise to the Contemporary Christian Music industry, which has grown by leaps and bounds since Petra first appeared in the early 1970s. While Petra have undergone several line-up changes through the years and shifted their sound in order to stay current with rock trends, the band's enormous popularity and its message have remained constant into the new millennium.