Complex sentence: Both Protestant and Roman Catholic communities have been in decline for some time now, due to the rise of irreligion in New England, the most irreligious region of the U.S., along with the Western United States.
Split into simpler sentences: Both Protestant and Roman Catholic communities have been in decline for some time now, due to the rise of irreligion in New England. It is the most irreligious region of the country, along with the Western United States.