ELGIN, Ill. — The small Church of the Brethren lost 1,707 members in 1999, representing a 1.2 percent membership drop for a church that now has only 138,304 people. But while the U.S. and Puerto Rican church saw its membership drop, the church’s sister body in Nigeria continues to grow. The Nigerian church now has between 130,000 and 140,000 members,…