Why does doctrine matter?

False doctrine is poison to the soul. An entire banqueting party drinking an admixture of arsenic can drink physical death from its cups. So an entire audience can invite spiritual and eternal death by listening to a sermon that contains an admixture of the poison of false doctrine. A person can be deprived of his soul’s salvation by a single false comfort or single false reproof administered to him.

—C. F. W. Walther, The Proper Distinction between Law and Gospel, page 20

That’s why we need the church “in which the Gospel is rightly taught,” as the Augsburg Confession puts it.