150–200

The formation of the New Testament canon, the rule of faith, and baptismal creeds

In response to Marcion's canon and the gnostics' secret traditions, the Church worked out its own answer in the second half of the 2nd century. It did so without any special council. The answer was open apostolic tradition attested by lists of bishops, and a gradually forming list of recognized writings.

By the end of the century the core of the New Testament canon had taken shape: the four Gospels, Acts, and the Pauline epistles. An early example of such a list is the Muratorian Fragment (c. 170). It excluded Hebrews, James and both epistles of Peter, but included the apocryphal Apocalypse of Peter.

In parallel, Rome developed a baptismal creed, the "Old Roman Symbol" (c. 150). It took the form of three questions about faith in the Father, Son and Spirit. By 200 it had been expanded with statements on the virgin birth, crucifixion under Pontius Pilate, and the resurrection of the flesh. These statements were aimed directly against docetism, Marcion and the gnostics.

This "Old Roman Creed" underlies the later Apostles' Creed.