Complex sentence: The party was overwhelmingly, almost exclusively Protestant and was accused of having links with the Orange Order causing it to be distrusted by Catholic voters and vigorously opposed by the Catholic Church who opposed the union not only for its alleged Orange links but for its trade unionism.
Split into simpler sentences: The party was overwhelmingly, almost exclusively Protestant, and was accused of having links with the Orange Order. As a consequence, it was distrusted by Catholic voters, and vigorously opposed by the Catholic Church, who opposed the union not only for its alleged Orange links, but also for its trade unionism.