So then, brothers, stand firm and hold to the traditions that you were taught by us, either by our spoken word or by our letter. 2 Thessalonians 2.15
The faith beyond the apostles
As we have seen, the Bible, both the Old and New Testaments, constitutes the founding document of the Kingdom of God. Given by God’s Spirit, it provides definition, framework, and operating principles by which men may enter the Kingdom of God and carry out the demands and privileges of their heavenly citizenship.
It would seem easy enough, therefore, for subsequent generations of the faithful simply to look to the Scriptures for answers to their questions and for guidance in navigating the uncertain waters of Kingdom living in the world. But it was not long after the passing of the apostles before differing voices arose, each claiming the authority of Scripture and using the language of the Gospel, but offering dramatically different interpretations of what it meant to follow Jesus.
How would the churches know whom to follow? Where could they look to gain a true and reliable interpretation of the Scriptures, so that they might be assured they were not believing in vain? The Church looked to her leaders – pastors, bishops, and theologians – to sort out these differences and to explain the true faith, once for all handed down to the saints. These leaders, in turn, looked to the apostles and to the work of the Spirit in the churches under their care as they labored to discern the continuity of teaching from the apostles to their present day. Then, summarizing these views, they began to publish them in polemical and apologetic works, designed to expose the errors of false teachers and to assert the true Gospel of the Kingdom with increasing clarity and breadth.
These leaders – men like Irenaeus and Tertullian – wisely sought to discern the most foundational teachings of the Spirit, which have come to be referred to as “rules of faith.” These became the foundation and touchstone for all orthodox teaching, and provided the theological and interpretive framework for the subsequent development of important creeds and confessions.
Irenaeus against the Gnostics
Irenaeus of Lyons (fl. 175-195) was the first to sketch the broad outlines of the true Gospel against the Gnostic teachings of the day. The Gnostics, using the language of Christian faith, espoused a sophisticated and elitist approach to believing, which blended elements of philosophy and pagan religion into a hierarchical system of faith. Some Gnostic sects insisted on a rigid asceticism, while others encouraged a more libertine approach to the flesh. All Gnostic sects depended on a kind of secret knowledge which was only attainable after long periods of initiation and, hence, made a separation among followers between those “in the know” and those who were simply ordinary believers.
Gnosticism undermined the Biblical teaching about God, Christ, the Spirit, the Trinity, and the life of faith in general, even as it used the terms of Scripture to press its false teachings.
In his work Against All Heresies Irenaeus considered the teachings of Gnosticism, demonstrating a masterful understanding of the heresy and dismantling its teachings in a thorough and systematic manner. In the process he set forth a brief summary of the true Gospel, which, he insisted, is what all true Christian churches believe and have believed from the days of the apostles. Given the thoroughness of his apologetic against Gnosticism, we can believe that Irenaeus was just as thorough in searching through the words of the apostles, the teachings of his forebears in the faith, and the understanding of his contemporaries on matters of Christian faith and practice.
The Rule of Faith
Irenaeus summarized the basics of true Christian faith as follows:
"…one God, the Father Almighty, who made the heaven and the earth and the seas and all things in them; and in one Christ Jesus, the Son of God, who became incarnate for our salvation; and in the Holy Spirit, who proclaimed through the prophets the dispensations and advents, and the birth from a virgin, and the passion, and the resurrection from the dead, and the incarnate ascension into heaven of the beloved Christ Jesus, our Lord….in order that to Christ Jesus, our Lord and God and Saviour and King, according to the will of the invisible Father, every knee should bow, of things in heaven, and things in earth, and things under the earth, and that every tongue should confess to him…" 
Irenaeus insisted that this teaching had been and was the rule of faith in all churches in every place:
"As I have already observed, the Church, having received this preaching, and this faith, although scattered throughout the whole world, yet, as if occupying but one house, carefully preserves it. She also believes these points of doctrine just as if she had but one soul, and one and the same heart, and she proclaims them, and teaches them, and hands them down, with perfect harmony, as if she possessed only one mouth."
What the Gnostics were teaching, therefore, was clearly out of line with what churches everywhere had believed and taught since the days of the apostles. Any teachers who might add to, diminish, or otherwise change this basic framework of faith should not be heeded, and any churches or believers embracing these views must reconsider and recant:
"For the faith being ever one and the same, neither does one who is able to discourse at great length regarding it, make any addition to it, nor does one, who can say little, diminish it."
Once for all
We can see in Irenaeus’ words seed thoughts of what will later become the Apostles’ Creed, the Nicene Creed, and the essential Trinitarian and Christological theology of the Church in every age. Irenaeus was not the first theologian to take up pen in the generation beyond the apostles, but he was among the first to mark out the “footprint” of Christian teaching as it relates to the bare essentials of what the followers of Christ must confess and obey.
His rule of faith provided a clarifying summary of the faith once for all delivered to the saints (Jude 3) and charted a course for subsequent creedal and doctrinal development. This rule of faith, and faithful statements of orthodoxy built upon it, provide an important secondary layer of Kingdom documents to guide the followers of Christ who must navigate uncertain philosophical, epistemological, and theological waters in every age. While not authoritative in the way that Scripture is, they are useful nonetheless as they guide the faithful in every age in a right understanding of the faith and the doctrines of Scripture. Failure to consult these secondary documents makes preachers, teachers, and theologians susceptible to individual whim or the temper of the age in trying to sort out the meaning and application of Christian faith to the demands of contemporary Kingdom living.
They will have more success in seeking the Kingdom and practicing the Kingship of Jesus who keep their belief and teaching squarely within the bounds of the rule of faith and of the historical course of Christian doctrinal development which has issued from it.
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Excerpts in J. Stevenson, ed., W. H. C. Frend, rev., A New Eusebius: Documents illustrating the history of the Church to AD 337 (London: SPCK, 1978), pp. 111, 112.