"Thanks be to your name," the pastor prays earnestly, his brow furrowed. "Be among us."

A chorus of amens bursts from the congregation. Some members have their hands raised. Onstage, young men and women in T-shirts and jeans launch into a ballad-like hymn of devotion, kicking off an hour and a half of often joyous, sometimes contemplative worship.

So begins one of thousands of weekly services in Protestant churches across Brazil. Although this largely tropical nation has more Roman Catholics than any other country in the world, it is witnessing a boom in evangelical Protestantism that could dramatically alter the religious landscape in the next 20 years. ...