Complex sentence: After that date, the relics of Saint James became an extraordinary centre of pilgrimage and from the 9th century have been kept in the heart of the church--the modern-day cathedral--dedicated to him.
Split into simpler sentences: After that date, the relics of Saint James attracted an extraordinary number of pilgrims. Since the 9th century these relics have been kept in the heart of the church--the modern-day cathedral--dedicated to him.