I think the happiness theory is the best answer. In Mohammad Yousuf's case converting to Islam got him greater happiness so he could perform better. I am sure there might be a lot of people in the world who gain happiness from Christianity. Some get happiness from totally absurd sources. Take me for example, everytime I drive my Mustang, I feel so happy. I bet if I were a cricketer I would scored several triple hundreds in the last year or so.