If music is the motor that drives Pentecostal worship, the band and choir at Ark of Salvation for the New Millennium, a little storefront church in west Harlem, could use a tuneup. Ragged and off-key at times, they are easily outclassed when they sit in with the seasoned musicians at other churches. Yet they grab attention for one simple reason: They are often the only teenagers in the room. As Pentecostalism advances across the world, winning converts faster than any other Christian denomination and siphoning believers from more established faiths, it is also suffering its own slow leak: young people...