Complex sentence: By the time Charles Calvert became governor, due to Puritan immigration, the population of the province had gradually shifted, becoming overwhelmingly Protestant, though political power tended to remain concentrated in the hands of the largely Roman Catholic elite.
Split into simpler sentences: By the time Charles Calvert became governor, the population of the province had gradually shifted due to Puritan immigration, becoming in time overwhelmingly Protestant. Political political power however tended to remain concentrated in the hands of the largely Roman Catholic elite.