In defending the truth, Irenaeus brought everything back to the past. That is, he tested everything against what had already been taught, what had been received, and what had been written down. He appealed, ultimately, to a “rule of faith”—a deposit of apostolic doctrine that had to be believed and should not be spoken against.

Guard the Deposit

The importance of orthodox theology in the early church cannot be overstated. Paul repeatedly tells Timothy to guard the deposit of apostolic truth entrusted to him (1 Tim. 6:20; 2 Tim. 1:13–14) and pass it on to others (2 Tim. 2:1–2). As a pastor in particular, Timothy must be able to teach (1 Tim. 3:2) and to correct his opponents with gentleness (2 Tim. 2:25). Similarly, Paul tells Titus he must be “able to give instruction in sound doctrine and also to rebuke those who contradict it” (Titus 1:9; cf. Titus 1:13). There is a core of apostolic teaching that the Christian church must embrace if it is to be Christian and if it is to be a church.

The Rule of Faith

Sadly, within some Christian traditions today, we see that doctrine is downplayed. We hear people talk about how right living (orthopraxy) is more important than right belief (orthodoxy) and how the Great Commission and the Great Commandment should caution us against spending too much time wrangling about doctrine. Almost all of us have heard the phrase (meant to be a good thing) that someone is “spiritual, not religious.” Even in evangelical churches, we too often settle for vague generalities. We are impatient with technical terms and careful reasoning. We prefer devotional platitudes instead of doctrinal precision.

But that’s not how the early Christians viewed their faith, at least not the ones who had enough education to write about their beliefs. Already in the second century, the church father Irenaeus (ca. 130–202) was referring to something called the “rule of faith.” And Irenaeus was only a couple of generations removed from the apostles. John had been Jesus’s disciple. He heard Jesus teach with his own ears; he saw the miracles with his own eyes; he was there on the Mount of Transfiguration, there at the empty tomb, and there in the upper room at Pentecost. This same John taught Polycarp (69–155), the famous (and ultimately martyred) bishop of Smyrna, who in turn taught Irenaeus.

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