On a recent episode of South Park, Cartman used a simple formula to climb the charts as the leader of a Christian rock group: Mimic the style of pop music, but for words like "baby," substitute words like "Jesus." Almost too accurate to be funny, the ploy worked for Cartman as well as it's worked for any number of contemporary Christian musicians. It also points to a tension (and an interchangeability) between the sacred and the sensual that has existed in contemporary music at least since Ray Charles borrowed church sounds to sing "I Got A Woman." Nobody has embodied that tension better than Al Green. The greatest soul singer of the '70s, Green met a series of personal misadventures that prompted him to chuck his pop career mid-decade in order to sing exclusively for the Lord. Green has back-pedaled on the commitment from time to time, usually with less-than-stirring results. (Theoretically, his voice can elevate any sound, but even he can't make a Diane Warren song soulful.) The new I Can't Stop reunites Green with classic-era producer Willie Mitchell, and returns him to Mitchell's Royal Studios, his home base during the years he recorded for Hi Records. It has all the makings of a classic album, but the ingredients don't quite mix right. Green's voice sounds great, swooping and dipping just like before. Every effort has been made to mimic the classic Hi sound, and it's admirable that Green and Mitchell wrote the songs themselves. But the results too often sound like an exercise, more pleasant than stirring. Maybe Green holds back on his old abandon out of respect for The Man Above, or maybe it's just impossible to pick up old work two decades later, but I Can't Stop could easily be a collection of not-quite-there studio rarities from Green's salad days. He's still got the fire inside, but it's turned from red to blue.