Irenaeus gives us the clearest explanation of the development of the Rule of Faith – it was developed to prepare for the Scriptures. Irenaeus writing for Lyons in modern-day France, states that the Rule of Faith was developed to preserve the ancient tradition without paper and ink for those without Scriptures.
1. Since therefore we have such proofs, it is not necessary to seek the truth among others which it is easy to obtain from the Church; since the apostles, like a rich man in a bank, lodged in her hands most copiously all things pertaining to the truth: so that every man, whosoever will, can draw from her the water of life. For she is the entrance to life; all others are thieves and robbers. On this account are we bound to avoid them, but to make choice of the thing pertaining to the Church with the utmost diligence, and to lay hold of the tradition of the truth. For how stands the case? Suppose there arise a dispute relative to some important question among us, should we not have recourse to the most ancient Churches with which the apostles held constant intercourse, and learn from them what is certain and clear in regard to the present question? For how should it be if the apostles themselves had not left us writings? Would it not be necessary, to follow the course of the tradition which they handed down to those to whom they did commit the Churches?
Irenaeus desires nothing to be add to what the Apostles had done, having given ‘all things pertaining to the truth’ to the Church already. The Gnostics were attacking the Church claiming true apostolic succession, but Irenaeus holds to the Rule of Faith as the tradition of the Apostles. Here he also makes the case to go to the source, the Apostles, to resolve any doctrinal disputes – not bishops or councils.
2. To which course many nations of those barbarians who believe in Christ do assent, having salvation written in their hearts by the Spirit, without paper or ink, and, carefully preserving the ancient tradition, believing in
one God, the Creator of heaven and earth, and all things therein, by means of Christ Jesus, the Son of God; who, because of His surpassing love towards His creation, condescended to be born of the virgin, He Himself uniting man through Himself to God, and having suffered under Pontius Pilate, and rising again, and having been received up in splendour, shall come in glory, the Saviour of those who are saved, and the Judge of those who are judged, and sending into eternal fire those who transform the truth, and despise His Father and His advent.
Those who, in the absence of written documents, have believed this faith, are barbarians, so far as regards our language; but as regards doctrine, manner, and tenor of life, they are, because of faith, very wise indeed; and they do please God, ordering their conversation in all righteousness, chastity, and wisdom. If any one were to preach to these men the inventions of the heretics, speaking to them in their own language, they would at once stop their ears, and flee as far off as possible, not enduring even to listen to the blasphemous address. Thus, by means of that ancient tradition of the apostles, they do not suffer their mind to conceive anything of the portentous language of these teachers, among whom neither Church nor doctrine has ever been established. (Irenaeus, Against Heresies, book 3, 4, 1-2)
Like the previous creeds, the One God is essential, but here, Irenaeus adds more detail to God and Son of God, in that we believe in the One God through Jesus Christ, bringing to our mind the words of Christ in John 14.6. Remember, Irenaeus was a disciple of Polycarp who was a disciple of the Apostle John.
This confession of faith was held by those who could neither read nor write the language of the Church at that time, but would be required to hold to the same doctrine. It was because of this ancient tradition as preserved in the Rule of Raith that the ‘barbarians’ were able to maintain the exact same doctrine which been established by the Apostles and given to the Church.