Complex sentence: Arnoldists were a kind of sectaries in the 12th century, named after Arnold of Brescia who criticized the great wealth and possessions of the Church, and preached against baptism and the Eucharist.
Split into simpler sentences: Arnoldists were a kind of sectaries in the 12th century, thus called from their chief, Arnold of Brescia. He declaimed much against the great wealth and possessions of the Church, and preached against baptism and the Eucharist.