An early Lutheran theologian quotes Martin Luther as saying that “justification is the article by which the Church stands or falls.” The closest we find in Luther’s own writings is a slight variation: that “if this article [of justification] stands, the church stands; if this article collapses, the church collapses.” Luther’s fellow Reformer John Calvin agreed, telling Cardinal Sadoleto that it was “the first and keenest subject of controversy” between Catholics and Protestants, and that “wherever the knowledge of it is taken away, the glory of Christ is extinguished, religion abolished, the Church destroyed, and the hope of salvation utterly overthrown.”
So according to the first Protestants, the doctrine of sola fide, that justification is by faith alone, isn’t just an important doctrine, it’s the make-or-break doctrine for Christianity (both in terms of individual salvation, and the survival of the Church). Given this, it’s surprising that the Protestant theologian and Church historian Philip Schaff should concede that anyone trying to find “the Protestant doctrine of justification by faith alone” being taught in the early Church “will be greatly disappointed.”
But if the early Christians weren’t the ones claiming that sola fide was “the heart of the Gospel,” where did this idea come from? The history is fascinating, and perhaps not what you might expect. For instance, did you know Martin Luther didn’t originally believe this doctrine, even in his first years as a Protestant? As Howard Griffith of Reformed Theological Seminary points out, “Luther’s doctrine of sola fide in 1520 is closer to ‘union with Christ by faith alone’ than to ‘justification by faith alone.’” So even when Luther used the expression “sola fide” (“faith alone”), he didn’t mean by it what later Protestants (including Luther himself) would mean.
Zondervan Academic (the Protestant publisher) explains that “in the early years of the Reformation (that is, before 1535), Luther did not make a sharp distinction between justification and sanctification,” believing (as Catholics do) that justification is “both an event and a process.” Further, Luther originally “did not necessarily characterize justification in forensic categories. His metaphors for justification tended to be of the marriage relationship or the medical process of healing.” In other words, Luther was initially very “Catholic” on the question of justification…even as a Protestant.
Only in the latter part of the 16th century do Protestants rally around sola fide in the sense that it is understood today. But even here, it wasn’t because this doctrine originally inspired the Reformers. In the words of Zondervan Academic, it’s because “this was the one thing all Protestants held in common and the thing that distinguished them from Rome.” That is, the reason sola fide became “the gospel” for Protestants wasn’t because the earliest Christians (including those who actually knew the Apostles) understood the gospel to mean that. They clearly didn’t. Instead, it was simply the only non-Catholic belief that Protestants could all agree on. It gave the Reformers something that both (a) rallied all Protestants together, and (b) allowed them to distinguish themselves from (and condemn) Catholics…even if it meant turning a novel doctrine into the “heart” of the gospel.