The Church is distinguished by four visible signs, called the marks of the Church. In the Formula of the Council of Constantinople (381) the Church is defined as
Unity – the one faith, one worship and one discipline under one supreme authority;
Sanctity – a holy People set apart for God’s service;
Catholicity – the universal, all‑embracing nature of the Church; and
Apostolicity – continuity with the apostles in teaching, governance and sacramental life.
These marks enable every soul to recognise the true Church of Christ.
Ask Aquinas is an AI assistant named for St. Thomas Aquinas, the Angelic Doctor. It answers questions about the Catholic faith from traditional sources, including:
The Summa Theologica of St. Thomas Aquinas
The Church Fathers, with the decrees of the Councils and writings of the Popes
The Catholic Encyclopedia (1907–1914)
The Douay-Rheims Bible, with Fr. Haydock's commentary
The Roman Catechism of the Council of Trent
Each answer cites the passages it draws on, so you can read them for yourself. This answer was reviewed by our team before it was published.
The sources carry ecclesiastical approval, but the AI's answers have not received a Nihil Obstat or Imprimatur and can contain mistakes. Ask Aquinas is not a substitute for the Magisterium of the Church, a priest or a spiritual director. For questions of conscience, preparing for the sacraments or your own spiritual life, please ask a priest.
