Complex sentence: The small Calvinist, Lutheran, and Hussite groups that sprang up were harshly persecuted by the Roman Catholic Church at first; in 1552 the Sejm suspended civil execution of ecclesiastical sentences for heresy.
Split into simpler sentences: The small Calvinist, Lutheran, and Hussite groups that sprang up were harshly persecuted by the Roman Catholic Church in their early years. Then in 1552 the Sejm suspended civil execution of ecclesiastical sentences for heresy.