Each of the early churches had a Rule of Faith. This was a set of basic doctrines learned and confessed at baptism, somewhat centered around a confession of the Trinity (Tertullian, Against Praxeas 11).
In the mind of the early churches Jesus had given the first rule of faith in Matthew 28:19: " … in the name of the Father, of the Son, and of the Holy Spirit."
Over the years, the churches had simply expanded Jesus' statement of faith as heresies had arisen. Tertullian described it as "a somewhat ampler pledge than the Lord has appointed in the Gospel" (De Corona 3).
Why did they do this?
It is important to understand the role tradition played in the early church. Protestants have rightly rejected the Roman Catholic view of the authority of tradition, but as is common in such cases, they lost an apostolic truth.
Tradition to the early churches was not the tradition of the Church or churches. Tradition was apostolic tradition.
The tradition that carried authority with the apostolic churches—the churches the apostles started themselves—was the body of tradition given to them by the apostles. Those churches believed it was their job—and, more specifically, the job of the elders—to preserve that tradition unchanged. Irenaeus, a noted bishop of the late 2nd century, explains the process as an answer to gnostic heresies:
We refer them to that tradition which originates from the apostles, which is preserved by means of the succession of elders in the churches. (Irenaeus, Against Heresies III:2:2, emphasis mine)
This is Scriptural. Paul said, "Stand fast and hold to the traditions which you have been taught, whether by word or by our letter" (2 Thess. 2:15, see also 1 Cor. 11:2). To Timothy, he added, "The things which you have heard of me among many witnesses, commit those to faithful men who will be able to teach others also" (2 Tim. 2:2).
This was apostolic tradition, and it was the goal of the early churches never to lose it.
As Jude said, "Contend earnestly for the faith once for all delivered to the saints" (v. 3).
You can find the many, many references to the rule of faith in the quotes section of this web site.
That's a thorough enough description of the rule of faith. Every church had one, and it was learned at baptism. There is one very interesting story that we ought to add.
Most people don't realize that the Nicene Creed is just Caesarea's rule of faith with small changes added by the council.
We know this from a letter written after the council to Caesarea in which Eusebius explained why he assented to the creed as given at Nicea. He explains:
We received from the bishops who preceded us, both in our instruction and when we were baptized—as we have also learned from the holy Scriptures—and in accordance with what we have both believed and taught while discharging the duties of elder and the episcopal office itself, so now we believe and present to you this distinct avowal of faith:
"We believe in one God, the Father Almighty, Maker of all things visible and invisible:
"And in one Lord, Jesus Christ, the Word of God, God of God, Light of light, Life of life, the only-begotten Son, born before all creation, begotten of God the Father, before all ages, by whom also all things were made; who on account of our salvation became incarnate, and lived among men; and who suffered and rose again on the third day, and ascended to the Father, and shall come again in glory to judge the living and the dead.
"We believe also in one Holy Spirit.
"We believe in the existence and subsistence of each of these: that the Father is truly Father, the Son truly Son, and the Holy Spirit truly Holy Spirit; even as our Lord also, when he sent forth his disciples to preach the Gospel, said, Go and teach all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Spirit."
Eusebius then adds that he told the emperor and the council:
Concerning these doctrines we steadfastly maintain their truth, and avow our full confidence in them … In the presence of God Almighty, and of our Lord Jesus Christ we testify, that thus we have believed and thought from our heart and soul …
We are moreover prepared to prove to you by undeniable evidences, and to convince you that in time past we have thus believed, and so preached. (as quoted by Socrates Scholasticus, Ecclesiastical History I:8)
There is a reason that this request came from the emperor and not the bishops.
It's clear from Eusebius' letter that he believed that every bit of his rule of faith was Scriptural. In fact, the bishops present at Nicea knew very well it was a difficult thing to add non-Scriptural terminology to the rule of faith (even if it might not be unscriptural).
To which doctrine, explained in this way, it appeared right to assent, especially since we knew that some eminent bishops and learned writers among the ancients have used the term homoousios in their theological discourses concerning the nature of the Father and the Son. (ibid.)
The bishops at Nicea were already on a slippery slope that would lead to a devastating fall of the Church. They had stars in their eyes as they looked at this emperor who had delivered them from persecution and made Christianity the favored religion of the empire.
Eusebius himself, even while struggling with adding a non-Scriptural word to the creed, referred to "the most pious emperor" as the one who suggested it. When he gives Constantine's explanation of the word, he refers to Constantine as "our most wise and pious sovereign."
We cover all this in our pages on the Council of Nicea. The point here is merely to say that until Nicea, the rule of faith adopted by the various churches always used Scriptural terminology.
The rule of faith was a rule learned at baptism by every believer. It is mentioned from the earliest of Christian writings. It expanded over time as heresies arose, but it remained rooted in Scripture until the Council of Nicea.
You can read several rules of faith and comments about the rule, also called the rule of truth, in the quotes section.
My favorite rule of faith is from Irenaeus in the late second century. It's my favorite because it's the most expanded, and therefore it has the most explanations of what's assumed in other, shorter ones. It's not different, it's just explained further.
It goes as follows. The first paragraph is his lightly expanded rule of faith, and the next two are his comments about unity:
The church, though dispersed throughout the whole world, even to the ends of the earth, has received from the apostles and their disciples this faith in one God the Father Almighty, Maker of heaven, earth, and the sea and everything 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, the advents, the birth from a virgin, the suffering, the resurrection from the dead, and the ascension into heaven in the flesh of the beloved Christ Jesus, our Lord, and his appearance from heaven in the glory of the Father to gather all things into one and to raise up anew all flesh of the whole human race, in order that every knee should bow—of things in heaven, things in earth, and things under the earth—and that every tongue should confess to him, and that he should execute just judgment towards everyone; that he may send spiritual wickednesses and the angels who transgressed and became apostates together with the ungodly, unrighteous, wicked, and profane among men into everlasting fire, but may, in the exercise of his grace, confer immortality on the righteous, holy, and those who have kept his commandments and persevered in his love—some from the beginning of their course and others from their repentance—and may surround them with everlasting glory.
As I have already observed, the Church, having received this preaching and this faith, although scattered throughout the world, yet, as if occupying but one house, carefully preserves it. She also believes these points just as if she had but one soul and one and the same heart. She proclaims them, teaches them, and hands them down with perfect harmony, as if she possessed only one mouth. For although the languages of the world are dissimilar, yet the understanding of the tradition is one and the same. For the churches which have been planted in Germany do not believe or hand down anything different, nor do those in Spain, nor those in Gaul, nor those in the East, nor those in Egypt, nor those in Libya, nor those which have been established in the central regions [i.e., the Middle East] of the the world. But as the sun, that creation of God, is one and the same throughout the world, so also the preaching of the truth shines everywhere and enlightens all men that are willing to come to a knowledge of the truth.
Nor will any one of the leaders in the churches, however highly gifted he may be in reference to eloquence, teach doctrines different from these, for no one is greater than his Master. Nor, on the other hand, will he who is deficient in powers of expression inflict injury on the tradition. For the faith being ever one and the same, neither does a person, who is able at great length to discourse regarding it, make any addition to it, nor does one, who can say but little, diminish it.
~ Against Heresies I:10:1