…within the Lutheran Church one meets people who, although they still belong to it, nevertheless look upon their Church as a kind of second rate Church, because they have a great respect for the outward prestige of the sectarian churches. The Lutherans in the General Synod, and in part, also those in the Council, consider themselves honored when they are treated by sectarian churches as a "denomination" of equal rank. Whether the sects recognize us or not, we should not think of ourselves as a second rate Church, nor allow ourselves to be pushed into a corner as outcasts who must first apologize for taking the liberty of existing alongside of sectarian church bodies. We should and must remain conscious of this — for God desires it of us —, that we alone, as the orthodox Church, have an express charter from God for our existence.
The sectarian church bodies exist only by God's permission. Therefore, our whole heart should belong to our orthodox Church. We should be glad to belong to it as a member, even under the most humble outward conditions; gladly serve it as preachers and teachers, and, with joyful spirit and restless zeal, work for its extension. We should most earnestly strive, and daily pray, that our descendants may be the heirs of the Church of the Reformation.
Luther confesses of himself, and of the whole church body named after him: "No man can deny this, that we have the ministry and God's Word pure and rich, that we use and teach it diligently, without any addition of our own new human doctrine, just as Christ commanded, and as the Apostles and all Christendom have done. We invent nothing new but rather hold to and remain with the old Word of God, as the old Church had it. Therefore, with it we are the true old Church, a one-of-the-same-kind Church, which teaches and believes the one Word of God. Therefore the papists again blaspheme Christ Himself, the Apostles, and all Christendom, when they call us Modernists and heretics. For they find nothing about us except only the same old things of the Ancient Church, and that we are like it and one Church with it." (Against Hans Wurst, 1541. E.A. 26,14.)
Luther states further: "The impurity of doctrine which is not God's Word, or is without it, is such a poisonous evil thing that even if St. Peter, yes, an angel from heaven would preach it, it would still be accursed, Gal. 1:8. Therefore false teachers and Anabaptists or fake masters of the Sacraments cannot be or remain in the Church, as Psalm I says, for they not only undermine the life, which the Church must bear, especially where this goes on under cover, but also the doctrine is undermined, which must openly be bright and shine, so that the life can be regulated according to it." (E.A. 26,37.)
from Franz Pieper, 'The Distinction Between Orthodox and Heterodox Churches'