What was "the rule of faith" the early Church appealed to?

Irenaeus of Lyons c. 130–c. 202 ADAgainst Heresies 1.10.1–2

The Church, though scattered through the whole world to the ends of the earth, received from the apostles and their disciples this faith: in one God, the Father Almighty, who made heaven and earth and the seas and all that is in them; and in one Christ Jesus, the Son of God, who became flesh for our salvation; and in the Holy Spirit, who proclaimed through the prophets the plan of God, the comings, the birth from the Virgin, the passion, the rising from the dead, and the bodily ascension into heaven of the beloved ... Having received this preaching and this faith, the Church, though scattered through the whole world, guards it carefully as if living in one house. ... The churches founded in Germany have not believed or handed down otherwise, nor those in Spain, nor among the Celts, nor in the East.

The fullest early statement of the rule of faith, kept in Greek through later writers. Its shape is the shape of the later creeds: Father, Son, Spirit.

Tertullian c. 155–c. 220 ADPrescription against Heretics 12

Let us seek in what is ours, from those who are ours, and about what is ours, and only what can be raised as a question without harm to the rule of faith.

The rule marks the limits of open questions. Inside it, Tertullian says, inquiry is free.

Tertullian c. 155–c. 220 ADOn the Veiling of Virgins 1.3

The rule of faith is altogether one, alone unchangeable and beyond reform: believing in one only God almighty, maker of the world; and in his Son Jesus Christ, born of the Virgin Mary, crucified under Pontius Pilate, raised from the dead on the third day, received into heaven, now seated at the Father's right hand, coming to judge the living and the dead through the resurrection of the flesh too. While this law of faith stays fixed, other matters of discipline and life admit correction, as the grace of God works and advances to the end.

Tertullian, by now in sympathy with the Montanists, distinguishes the fixed rule of faith from church discipline, which he thinks can develop.

Origen of Alexandria c. 185–c. 254 ADOn First Principles Preface 2

Since many who think they hold the things of Christ think differently from those before them, let the Church's preaching be kept, handed down from the apostles by the order of succession and remaining in the churches to the present. That alone is to be believed as truth which differs in nothing from the Church's and the apostles' tradition.

Origen then lists the points the apostles taught plainly and leaves other questions open for study. Survives in Rufinus's Latin.

Novatian fl. c. 250 ADOn the Trinity 9.1

The same rule of truth teaches us to believe, after the Father, also in the Son of God, Christ Jesus, our Lord God, but the Son of God: of this God who is one and alone, the maker of all things, as has been said above.

Novatian, a Roman presbyter, organizes his whole treatise as a commentary on the rule of truth.

Augustine of Hippo 354–430 ADOn Christian Teaching 3.2.2

When careful attention finds it uncertain how a passage should be punctuated or pronounced, let the reader consult the rule of faith, which he has learned from the plainer places of Scripture and from the authority of the Church.

Augustine draws the rule from Scripture's clear passages and from the Church together.