Complex sentence: During the Reformation a number of new dioceses were founded, but no more were then created until the middle of the 19th century, when dioceses were founded mainly in response to the growing population, especially in the northern industrial cities.
Split into simpler sentences: During the Reformation a number of new dioceses were founded. No new dioceses were then created until the middle of the 19th century, when dioceses were founded mainly in response to the growing population, especially in the northern industrial cities.