[Martin] Luther called justification by faith alone “the article upon which the church stands or falls” (articulus stantis et cadentis ecclesiae). This strong assertion of the central importance of justification was linked to Luther’s identification of justification by faith alone (sola fide) with the gospel. The “good news” of the New Testament includes not only an announcement of the person of Christ and his work in our behalf, but a declaration of how the benefits of Christ’s work are appropriated by, in, and for the believer.
The issue of how justification and salvation are received become the paramount point of debate. Luther’s insistence on sola fide was based on the conviction that the “how” of justification is integral and essential to the gospel itself. He viewed justification as one involving an essential truth of Christianity, a doctrine no less essential than the Trinity or the dual natures of Christ. Without the gospel the church falls. Without the gospel the church is no longer the church.
The logic followed by the Reformers is this:
1. Justification by faith alone is essential to the gospel.
2. The gospel is essential to Christianity and to salvation.
3. The gospel is essential to a church’s being a true church.
4. To reject justification by faith alone is to reject the gospel and to fall as a church.
The Reformers concluded that when Rome rejected and condemned sola fide, it condemned itself, in effect, and ceased to be a true church.
taken from: Faith Alone: The Evangelical Doctrine of Justification, Baker Book House, 1995, pp. 18-19, emphasis added.