Complex sentence: Louis was a key figure in the revolt of the Netherlands against Spain and a strongly convinced Calvinist, unlike his brother William, whom he helped in various ways, including by arranging the marriage between him and his second wife Anna of Saxony.
Split into simpler sentences: Louis was a key figure in the revolt of the Netherlands against Spain, unlike his brother William, he was a strongly convinced Calvinist. He helped his brother in various ways including by arranging the marriage between William and his second wife Anna of Saxony.