Complex sentence: The most treasured possession of the church were the supposed skulls of Saints Andrew, Luke and Timothy, but the church also held relics of Saint John Chrysostom and other Church Fathers, saints and martyrs.
Split into simpler sentences: The most treasured possession of the church were the supposed skulls of Saints Andrew, Luke and Timothy. But the church also held relics of Saint John Chrysostom and other Church Fathers, saints and martyrs.