Karl Barth, CD I/2, pp. 878-879, as quoted by George Hunsinger, in his "Election and the Trinity: twenty-five theses on the theology of Karl Barth," Modern theology 24, no. 2 (April 2008): 189, but with the Thomson/Knight translation restored (since Hunsinger, or perhaps just Modern theology, drops an ellipsis).
The stress in context is on "revelation," or God's "work and activity" ad extra, "the actuality of the Word of God" as distinguished from "the doctrine of the Trinity"considered as a "dogma," "doctrine," "basic view, or . . . controllable principle which can be used as such for the construction of a system. . . . of trinitarian doctrine."
We did not derive our differentiation of the Loci [into De creatione, De reconciliatione, De redemptione (and before those De Deo)] from the doctrine of the Trinity. We derived the doctrine of the Trinity itself from the same source as that from which is now derived the differentiation of the Loci, viz., the work and activity of God in His revelation.
The doctrine of the Trinity, like all other doctrines, is preceded by the fact of revelation in itself and as such. The essence of this fact is that God confronts us as Creator, Mediator, and Redeemer, that as such He speaks and deals with us, that He is therefore God and Lord in this threefold way. This being of God in His work and activity is not a dogma, or a basic view, or a controllable principle which can be used as such for the construction of a system. It is the actuality of the Word of God, freely preceding and underlying all views and dogmas. We attained our differentiation of the Loci by reference to this actuality and not to the doctrine of the Trinity, although inevitably [this actuality] both confirms the latter, and is itself confirmed by it, and safeguarded against misunderstandings.Only with what I've just placed in italics do we arrive at the use Hunsinger makes of this passage. This use is perfectly legitimate, as indeed Barth himself makes abundantly clear
("In §§ we thought it necessary to understand God in His revelation as Creator, Mediator and Redeemer in order to see as the foundation of this threefold division of His self-revealing action the fact that in Himself and to all eternity God is Father, Son and Holy Spirit" (and then very shortly the first of the two sentences quoted by Hunsinger)),but not the main point here.
The dangers Hunsinger sees in the new revisionism of McCormack and company were dangers for the substitution of Creator-Redeemer-Sustainer before that.