"With the utmost rigor we demand that all of the articles of Christian doctrine . . . be kept pure and certain. This is supremely necessary. For this doctrine is our only light, which illuminates and directs us and shows the way to heaven; . . . We can be saved without love and concord [among churches] . . . but not without pure doctrine and faith. . . . Therefore, as I often warn you, doctrine must be carefully distinguished from life. . . . [W]e can be lenient toward errors of life. For we, too, err daily in our life and conduct; so do all the saints, as they earnestly confess in the Lord's Prayer and the Creed. But by the grace of God our doctrine is pure; we have all the articles of faith solidly established in Sacred Scripture. The devil would dearly love to corrupt and overthrow these; that is he attacks us so cleverly with this specious argument about not offending against love and the harmony among the churches."
Martin Luther, Lectures on Galatians (1535) 5:10 ("and he who is troubling you will bear his judgment, whoever he is"), trans. Jaroslav Pelikan = LW 27, 41-42 = WA 40, p. 51 l. 15-p. 52 l. 25, as quoted by Carter Lindberg, in his "Luther's critique of the ecumenical assumption that doctrine divides but service unites," Journal of ecumenical studies 27, no. 4 (Fall 1990): 684 (679-696).
"We can be saved without love and concord" is going too far (cf. e.g. the "faith which worketh by charity" of the Decree on justification (Trent VI.7)), but there is something to the point Luther is making here nonetheless.
"Doctrine and life are to be distinguished. Life is as bad among us as among the papists. Hence, we do not fight and damn them because of their bad lives. Wyclif and Hus, who fought over the moral quality of life, failed to understand this. I do not consider myself to be pious. But when it comes to whether one teaches correctly about the Word of God, there I take my stand and fight. That is my calling. To contest doctrine has never happened until now. Others have fought over life; but to take on doctrine—that is to grab the goose by the neck! . . . [W]hen the Word of God remains pure, even if the quality of life fails us, life is placed in a position to become what it ought to be. That is why everything hinges on the purity of the Word. I have succeeded only if I have taught correctly" (WA, Tischreden 1, pp. 294-295 (#624), as quoted by Lindberg (683), quoting Ozment; Lindberg, at least, gives no indication what a mishmash of Latin and German this is in the original!).
"It is not a fruit of the Spirit to criticize a doctrine by the imperfect life of the teacher. . . . I am not so much offended by the unfruitfulness of the spirit of Allstedt [Müntzer] as I am by his lying and his attempt to establish other doctrines. I would have paid little attention to the papists, if only they would teach correctly. Their evil life would not cause much harm. . . . For the Spirit of Christ condemns no one who teaches rightly, but bears with and supports and helps those who live rightly. He does not despise weak sinners as this Pharisaical spirit [Müntzer] does" ("Letter to the Princes of Saxony concerning the rebellious spirit" (1524), LW 40, 57 (49-59), as quoted by Lindberg (684); cf. WA 15, 218 (210-221)).
But Luther wasn't always consistent: "John Huss preached thus against the Pope from Matt. xvi. 18 — 'The gates of hell shall not prevail against my church. Is there there any obscurity or ambiguity? But the gates of hell do prevail against the Pope and his, for they are notorious throughout the world of their open impiety and iniquities. Is there any obscurity here either? ERGO: THE POPE AND HIS, ARE NOT THE CHURCH CONCERNING WHICH CHRIST SPEAKS'" (De servo arbitrio 37, trans. Henry Cole (discussion of the perspicuity of Scripture) =WA 18, 657, ll 21 ff.; note that there is no emphasis in the original as reproduced in the Weimarer Ausgabe).