Complex sentence: Until the thirteenth century, the Apostles'Creed and the Lord's Prayer formed the general basis of religious instruction; all the faithful within the Catholic Church had to know them by heart, and parish priests were commanded to explain them on Sundays and festivals.
Split into simpler sentences: Until the thirteenth century, the Apostles'Creed and the Lord's Prayer formed the general basis of religious instruction. All the faithful within the Catholic Church had to know them by heart, and parish priests were commanded to explain them on Sundays and festivals.