Ernst Käsemann argues that the New Testament scriptures have been subject to biblical criticism from their very beginning, and even the New Testament writers criticize each other, disagree with each other and contradict each other. In Käsemann's essay "Is the Gospel Objective?" in his Essays on New Testament Themes, he argues that the Matthew and Luke "deliberately alter" Mark's gospel for "dogmatic reasons" in their respective gospels on a "hundred occasions"! (The consensus of New Testament theologians and scholarship today is that Matthew and Luke used Mark's gospel as a source in their gospels, c.f. the Two-Source Hypothesis). To emphasis this point, skim through a parallel of these synoptic gospels and see how the gospels of Matthew and Luke radically alter their source text from the gospel of Mark, demonstrating that scriptures such as the gospel of Mark were not treated as unchangeable source texts impervious to all biblical criticism, but instead and from the very beginning, and even within Scripture itself, biblical criticism was taking place. If Mark may be changed on a hundred occasions for dogmatic reasons, then all scripture is subject to be changed on in a hundred locations for dogmatic reasons likewise!

Ernst Käsemann writes:

The phenomenon of biblical criticism has been a characteristic of Protestantism. This seems strange, for it is Protestantism which proclaims the sole sovereignty of Scripture and yet explains its its own existence by reference to the same datum. For it is never permissible to speak of this sole sovereignty as a formal principle, and although we have been doing this for a long time now. This statement needs clarification. Unquestionably biblical criticism has always existed. When Matthew or Luke deliberately alter, for dogmatic reasons, the text of Mark which they have before then (as they do on a hundred occasions), we detect there the presence of a very early form of biblical criticism. Very soon it is joined by a second, stemming less from Dogmatics than from historical-philological scholarship; we can probably see a good example of this in the variants contained in the Codex D version of Acts. [1].

Käsemann anticipates a possible objection that only the biblical authors were allowed to engage in biblical criticism. This objection is answered by Käsemann's example of "Codex D", which is also known as Codex Bezae because it was once owned by John Calvin's student Theodore Beza. Käsemann explains that biblical criticism is also evident in the transmission of biblical manuscripts, and gives the example of Acts from Codex Bezae, which is a 5-6th century biblical manuscript in the Western text-type family, and it is about "ten percent longer" [2] than the older and best known manuscripts.

Textual criticism has discovered that biblical manuscripts have not be transmitted uniformly, but have diverged into different text-type families, that have a unique set of variants depending on the regions from which they originated and were transferred throughout the world. When scribes copied biblical manuscripts, they compared these various forms of these manuscripts from different text-type origins, and then pick variants for dogmatic reasons, and they often picked some verses from one manuscript source and other variants from other manuscripts, to create new versions of the books of the bible that had never existed collectively in any previous manuscript.

Even today in the newest and best New Testament Greek bibles such as the NA28 and UBS5, contain an eclectic text assembled from thousands of manuscripts, and there is no one ancient greek manuscript that exactly represents the majority of pages in these Greek NT bibles, and from these sources all english translations originate! This is another powerful example of biblical criticism that allows for the intentional alteration of the scriptures.  

Käsemann concludes from the examples of Matthew's and Luke's alteration of Mark, and Codex D's verison of Acts, that our received New Testament scriptures are not above biblical criticism, and that these differences in opinions between the syntopic gospel authors and the scribes that transmitted their gospels to us, that the New Testament canon must be tested, in the same fashion as we test any spirit (c.f. 1 John 4:1) to determine whether the gospel has been best expressed and rightly transmitted to us. Käsemann concludes with language similar to Karl Barth that the scriptures are not the Word of God, but they do reveal the Word of God, and concludes similar to Rudolf Bultmann that scriptures do not give us objective and direct access to the content they contain, but we may truly access it through the rise in faith that comes from our act of decision.

Ernst Käsemann writes:

We are, in other words--indeed, in New Testament language--driven to test the spirits even within the Scripture itself. We cannot simply accept a dogma or a system of doctrine but are placed in a situation vis-a-vis Scripture which is, at the same time and inseparably, both responsibility and freedom. Only to such an attitude can the Word of God reveal itself in Scripture; and that Word, as biblical criticism makes plain, has no existence in the realm of the objective--that is, outside our act of decision. [3]

In the same essay, Käsemann provides a helpful reconstruction of the Protestant scripture principle that defines the priority of scripture in terms of a dialectic between the gospel and the New Testament, in a way that avoids the errors of enthusiasm or traditionalism. I highly recommend the entire essay. 

Sources:

1. Ernst Käsemann, Essays on New Testament Themes, "Is the Gospel Objective?", trans. W. J. Montague, (SCM Press, 1968: Great Britain), p. 54

2. Bruce Metzger, A Textual Commentary on the Greek New Testament, (Biblia-Druck, 1971: Stuttgart), Introduction, p. 6* 

3. Ibid. Käsemann. p. 58. 

4. Background of header image: source wikipedia