Sources: Vatican I decrees (1870 — papal infallibility); Vatican II documents (1962–65 — Lumen Gentium, Gaudium et Spes); Aquinas, Summa Theologiae; Trent decrees (1545–63).

Roman Catholicism as a distinct entity (post-1054) is defined against Eastern Orthodoxy and later against Protestantism. Key theological distinctives: (1) Papal primacy and infallibility — Vatican I (1870) defined that the Pope, when speaking ex cathedra on faith and morals, is infallible; (2) The seven sacraments — baptism, Eucharist, confirmation, reconciliation, anointing of the sick, holy orders, matrimony — all as efficacious means of grace (ex opere operato); (3) Transubstantiation — the Council of Trent (1551) defined that the bread and wine become the body and blood of Christ, with the substance changing while accidents remain; (4) Purgatory — a state of purification after death, with the living able to aid the souls there through prayer and indulgences; (5) The deuterocanon — the Catholic Bible includes seven books (Tobit, Judith, 1–2 Maccabees, Wisdom, Sirach, Baruch) not in the Jewish or Protestant canon; (6) Natural law tradition — Thomistic philosophy provides the basis for Catholic moral teaching on bioethics, sexuality, and social justice. Vatican II (1962–65) dramatically updated Catholic engagement with modernity: religious freedom, ecumenism, the liturgy in vernacular, and a positive relationship with Judaism (Nostra Aetate).