Complex sentence: From 1697 the Kings of Saxony in order to accept the crown of Poland which they held until 1763, were Roman Catholic, ruling over a domain that was 95% Protestant.
Split into simpler sentences: From 1697 the Kings of Saxony became Roman Catholic in order to accept the crown of Poland, of which they were kings until 1763. The royal family remained Roman Catholic, ruling over a domain that was 95% Protestant.