Each doctrine was hammered out — often across centuries and against fierce opposition — in the writings of theologians and the decrees of councils.
One God in three co-eternal, consubstantial persons: Father, Son, and Holy Spirit.
The eternal Word became flesh — one person in two natures, fully divine and fully human, without confusion or division.
How the sinner is made right with God — a central fault line between Catholic and Protestant theology since the Reformation.
The presence of Christ in the consecrated bread and wine, defined at Lateran IV as transubstantiation.
The nature and limits of the Bishop of Rome's teaching authority, defined at Vatican I under the doctrine of infallibility.
Sacred images may be venerated (proskynesis), though not worshipped (latria) — a distinction restored at Nicaea II.
The relationship of divine grace and human freedom, contested by Pelagius, Augustine, Luther, and the Council of Trent.
The right of every person to seek truth without coercion, formally affirmed by the Second Vatican Council in Dignitatis Humanae.