Complex sentence: Most of his later life was spent defending his doctrine against Catholic theologians who held that Catholicism came from the apostles and that Pelagius was spreading novelties in the Faith unknown to the apostolic tradition.
Split into simpler sentences: Most of his later life was spent defending his doctrine against other Catholic theologians. They held that Catholicism came from the apostles and that Pelagius was spreading novelties in the Faith unknown to the apostolic tradition.