Complex sentence: His career suffered due to suspicions that he was secretly a Roman Catholic, due partly to his marriage into a famous recusant family, and partly due to the indifference which both he and his wife showed to observance of the Anglican faith.
Split into simpler sentences: His career suffered due to suspicions that he was secretly a Roman Catholic. These suspicions were due partly to his marriage into a famous recusant family, and partly due to the indifference which both he and his wife showed to observance of the Anglican faith.