The Congregation for the Doctrine of the Faith presents ecclesial communion as a key to understanding the Church and renewing theology, linking the images of the People of God, the Body of Christ, and the Church as sacrament. Communion unites believers vertically with the Trinity and horizontally with one another through Christ, faith, Baptism, and especially the Eucharist, while embracing both the Church’s visible institution and its spiritual fulfillment. The Church is consequently a sign and instrument of humanity’s unity and a mission-oriented community. Universal and particular Churches are not a federation of self-sufficient communities: they exist in “mutual interiority,” and every Catholic belongs immediately to the universal communion. Eucharistic unity is inseparable from the Episcopate, apostolic succession, and the Petrine ministry, whose supreme and immediate authority is present within every particular Church. Diversity of cultures, ministries, religious communities, and traditions enriches unity when ordered to communion. Other baptized Christians share an imperfect but real communion, especially the Eastern Orthodox, while ecumenism requires prayer, penance, dialogue, and collaboration directed toward full visible unity. Mary is presented as the model and intercessor of ecclesial communion.