Complex sentence: Two years later the Diet made a Romanian Calvinist pastor the head of the Church of the Romanians in John Sigismund's realm, ordering the expulsion of the Orthodox clerics who were unwilling to adhere to Calvinism, but the decree was actually never implemented.
Split into simpler sentences: Two years later the Diet made a Romanian Calvinist pastor the head of the Church of the Romanians in John Sigismund's realm. The Diet also ordered the expulsion of the Romanian Orthodox clerics who were unwilling to adhere to Calvinism, but the decree was never implemented.