Luther had a dynamic form of Christian life. Zwingli and, as we shall see, Calvin also had a static one: faith is psychological health. If you are psychologically healthy, then you can have faith, and vice versa. They are identical, actually. Faith for Luther is a dynamic thing, going up and down, reaching heights and depths. This is always possible for Luther. For Zwingli it is much more static, much more humanistically balanced. It is much more something which is similar to the bourgeois ideal of health. Faith is psychological health. “Christian faith is a thing which is felt in the soul of the faithful like health in the body.” The continuous breaking down and re-arising of the community with the personal, wrathful and loving God, is Luther’s type. The corresponding undynamic union with God is Zwingli’s type.
Zwingli is progressive; Luther is paradoxical. And therefore it is so difficult to speak about the paradox, on Zwinglian soil. Either the paradox is dissolved or, if not, then it is accepted. The paradox of the Christian life against the rational progressivism of the Christian life this is the basic difference. But there is another difference to which we shall go tomorrow.
Paul Tillich, A History Of Christian Thought – Table of Contents