In the chapter on The Spirit in Evangelical Theology: An Introduction it is interesting how much Barth cautions about relying on presuppositions in theology. In particular, such a reliance can rob Theology of real power, that of the Holy Spirit.
If theology wished to provide a presupposition for its statements, it would mean that it sought to make them, itself, and its work safe from any attack, risk, or jeopardy. It would presume that it could and must secure them (even if this presupposition was a tour de force, a Deus ex machina introduced in the form of a further theological statement). Precisely in this way theology would sell its birthright for a mess of pottage. Theology can only do its work. It cannot, however, seek to secure its operation. Its work can be well done only when all presuppositions are renounced which would secure it from without or within.
What can be arbitrarily presupposed, obviously stands at one's disposal. Were theology to presuppose the power sustaining its statements and itself (in the manner that mathematics presupposes the axioms supporting its theorems), then theology would assume power in its own right, superior to that first and fundamental power. Theology could then muster that power for its self-protection or at least place it on guard duty.
The true power, which is powerful in its own right, defies being a potency which theology can possess and manipulate in its statements. Such presumed potency would be something like Münchausen trying to pull himself out of the bog by his own hair. In one way or another the very thing theology seeks (because in fact it needs it) would be lost whenever theology attempted to rely upon such an arbitrary presupposition.
We have to speak, therefore, of the real power that is hidden in theological assertions-hidden, unattainable, unavailable not only to the environment but also to the very theology which serves the community. This is the power present and active in what the affirmations of theology declare, in the history of salvation and revelation, in the hearing and speech of the biblical witnesses, in the being and act of the community summoned by them, and also in the work of theology when it testifies to these things. But this power is also totally superior to theology itself.Evangelical Theology, page 50-51.
The Holy Spirit is the vital power that bestows free mercy on theology and on theologians just as on the community and on every single Christian....
In its total poverty evangelical theology is rich, sustained, and upheld by its total lack of presuppositions.page 59