Protestants confess sola scriptura, the Scriptures as the final rule of faith and practice. “Reformation by the word of God” is the watchword of Protestantism.
The Scriptures that are our final rule teach us that the church is one, that she ought to be in reality what God has declared her to be, and that this unity must take the visible form of love.
Here is one of the primary areas where Protestantism has to be reformed by the word of God. When Protestantism turns sectarian, it denies Scripture. Sectarian Protestantism is a denial of Protestantism.
Sola scriptura and una ecclesia are inseparable.
Catholicity is the only option for a Protestantism that takes sola scriptura seriously.