Barth again develops a contrast with Lutheranism. In many ways the Lutheran Church is the church of the 'material principle' of the Reformation - justification by grace alone through faith alone in Christ alone. This is the organising 'idea' of Lutheran theology, and it is noticeable that this has an effect on the way the Lutherans approach the Bible. Consider Luther's attitude to James, for example: it doesn't teach justification by faith alone, therefore it is secondary, unexciting. This also allows for the privileged position accorded in Lutheranism to the Augustana and to the work of Luther generally.
The Reformed churches, on the other hand, took up the 'formal principle' as their particular emphasis. That ruled out the possibility that they should become 'Calvinist churches' or 'Zwinglian churches' in the same sense as the Lutheran Church. Scripture alone also ensured that the Reformed confessions took on a very different role to the Augustana. The Reformed churches could only see their confessions as pointing to Scripture. They were not the light, but they pointed to the light (Barth develops the analogy with John the Baptist, an important one for his theology generally). In essence, Reformed Christianity is simply this attitude to Sola Scriptura.
He goes on to trace the idea of the grounds of this principle. In Calvin, the grounds of the Scripture Principle is simply the Spirit speaking in Holy Scripture. The Bible is God's Word because God address me in it. The Spirit in me and the Spirit in the Word are one. The early Reformed confessions generally take this line. Even at this early stage - and even in Calvin - it is acceptable to append arguments from the style or circumstances of Scripture, but they are understood as just that: appendices. The main point is Inspiration, and the witness of the Holy Spirit.
The sad history of the Reformed churches is a move away from this basis in two directions. Firstly, there is a tendency to make the arguments which had been appended to the Scripture Principle the real basis for taking Scripture as God's Word. A loss of confidence in the basis of the Principle in the doctrine (and experience!) of the Holy Spirit led to more emphasis on the arguments, and eventually to the arguments taking over. Scripture is made subject to the judgement and reason of human beings.
Secondly, there is a tendency to make the Bible just one thing amongst many others. Obviously, this goes hand in hand with the first. There is a movement away from seeing God revealed only in the Bible, to seeing the Bible as merely the pinnacle of God's revelation in creation and the human spirit.
In both cases, the door is opened to Protestant liberalism and modernism. That door is shut again only when we say that the Word of God in Scripture is self-authenticating. God witnesses to God. The Word of God is not chained, but speaks clearly and powerfully by the Holy Spirit.