Are there doctrines that Lutherans have to believe in? Like the creeds? Luther in his conflict with Erasmus states: “Take away assertions and you take away Christianity.” Yes the movement from assertions to faith is not a direct one. As Dr. Gerharde Forde points out here, everything depends on how assertions are used. The Lutheran concern is not for the infallibility of propositional truths. Rather, the Lutheran concern is for pure doctrine – doctrine not contaminated by human systems of legitimation. There is no pure church; there is pure gospel.
Gerhard O. Forde, “The Early Lutheran Tradition,” in Teaching Authority and Infallibility in the Church, Lutherans and Catholics in Dialogue 6, 1978.