On “justification by grace through faith”:
A word must be said about the expression “Justification by grace through faith.” It is often used in the abbreviated form of “Justification by faith.” But this is extremely misleading, for it gives the impression that faith is an act of man by which he merits Justification. This is a total and disastrous distortion of the doctrine of Justification. The cause is God alone (by grace), but the faith that one is accepted is the channel through which grace is mediated to man (through faith). (Systematic Theology, vol. 2, p. 179)
On the “Protestant principle”:
How can a faith which has doubt as an element within itself be united with creedal statements of the community of faith? The answer can only be that creedal expressions of the ultimate concern of the community must include their own criticism. It must become obvious in all of them–be they liturgical, doctrinal or ethical expressions of the faith of the community–that they are not ultimate. Rather, their function is to point to the ultimate which is beyond all of them. This is what I call the “Protestant principle,” the critical element in the expression of the community of faith and consequently the element of doubt in the act of faith. Neither the doubt nor the critical element is always actual, but both must always be possible within the circle of faith. From the Christian point of view, one would say that the Church with all its doctrines and institutions and authorities stands under the prophetic judgment and not above it. Criticism and doubt show that the community of faith stands “under the Cross,” if the Cross is understood as the divine judgment over man’s religious life, and even over Christianity, though it has accepted the sign of the Cross. (Dynamics of Faith, p. 33)
Needless to say, contemporary Protestant churches frequently fall short both by treating faith as a “work” we perform to earn God’s favor and by absolutizing expressions of their faith–doctrinal, moral, institutional, or whatever. But the Reformation message of God’s free and unconditional grace is meant to free us from reliance on our works–including our religious works–and our tendency to turn them into idols.