Eastern Christianity and are Churches that have always remained in full communion with the Catholic Church or who have chosen to reenter full communion in the centuries following the EastWest Schism and earlier divisions. 44 In the Catholic Church, universal positive ecclesiastical laws, based upon either immutable divine and natural law, or changeable circumstantial and merely positive law, derive formal authority and promulgation from the office of pope who, as Supreme Pontiff, possesses the totality. Aquinas' Summa Theologica was an intellectual milestone in its synthesis of the legacy of Ancient Greek philosophers such as Plato and Aristotle with the content of Christian revelation. During this period, monastic Ireland became a centre of learning and early Irish missionaries such as St Columbanus and St Columba spread Christianity and established monasteries across continental Europe. "Responses to Some Questions Regarding Certain Aspects of the Doctrine of the Church". These roles, Brown says, "contributed enormously to seeing the bishop of Rome, the bishop of the city where Peter died, and where Paul witnessed to the truth of Christ, as the successor of Peter in care for the church universal". 142 Sacraments Main article: Sacraments of the Catholic Church Mass at the Grotto at Lourdes, France. Many Catholic pastors and priests continued to be sent to prison for refusing to renounce allegiance to Rome.