Complex sentence: Williams was a confirmed atheist, his wife a devout Catholic--placed a strain on their relationship, which reached breaking point when Williams had an affair with Patricia Law Skinner, then wife of the historian Quentin Skinner.
Split into simpler sentences: Williams was an atheist, his wife a Catholic--placed a strain on their relationship. It reached breaking point when Williams had an affair with Patricia Law Skinner, then wife of the historian Quentin Skinner.