VATICAN CITY (CNS) -- Pope Benedict XVI urged American Catholics to strive for greater unity, especially among ethnic groups and between bishops and religious orders, in order to carry out the church's mission in an increasingly hostile society.The pope made his remarks May 18 in a speech to U.S. bishops from the Chaldean, Ruthenian, Maronite, Ukrainian, Armenian, Melkite, Syriac and Romanian Catholic churches, who were making their periodic "ad limina" visits to the Vatican.They were the last of 15 groups of U.S.