“They understood not the saying, which he spake unto them.”
Luther, Commenting on Luke 2:41-52 (Second Sermon)
32. This should shut the mouths of vain babblers who exalt the holy Virgin Mary and other saints as if they knew everything and could not err; for you can see here how they err and falter, not only in this that they seek Christ and know not where to find him until they accidentally come to the temple, but also that they could not understand these words with which he censured their ignorance and is compelled to say to them: “Knew ye not, that I must be in the things of my Father?” The Evangelist has pointed this out with great diligence, in order that men should not give credence to such falsehoods as ignorant, inexperienced and conceited teachers of workrighteousness present in exalting the saints, even setting them up as idols.
38. ...[T]hey have abandoned Holy Scripture; yet so as to attach it to some of the writings and expositions of the fathers, nevertheless not any farther than it pleased the pope and would not prove contrary to his law, and that no one should use Scripture except in accordance with the pleasure of the pope, to whom alone pertains the interpretations of Scripture and whose knowledge and judgments every one is bound to accept. Yet, in spite of this, they so far honor the fathers as to demand that their interpretations and explanations should be followed. All the world accepted this and so received all that the fathers said, as if they could not err, and shouted again: Aye, how could it be possible that so many holy, learned and highly intelligent men should not have understood the Scriptures?
39. To this we should reply as is taught in this Gospel: Be they called holy, learned, fathers, councils, or any other name, even though they were Mary, Joseph and all the saints it does not follow that they could not have erred and made mistakes. For here you learn that the mother of Christ though she possessed great intelligence and enlightenment, showed great ignorance in that she did not know where to find Christ, and in consequence was censured by him because she did not know what she should have known. If she failed and through her ignorance was brought to such anxiety and sorrow that she thought she had lost Christ, is it a wonder that other saints should often have erred and stumbled, when they followed their own notions, without the guidance of Scripture, or put their own notions into Scripture.
46. You say further: Yea, the church and the fathers were endowed with the Holy Spirit, who kept them from error. The answer to this is not difficult: The church and councils may have been ever so holy, they did not have the Holy Spirit in greater measure than Mary, the mother of Christ, who was also a member, yea, at the time, the most eminent member of the Church. And although she had been sanctified by the Holy Spirit; yet he permitted her at times to err, even in the important matters of faith. From this it does not follow, that the saints, who were endowed with the Spirit, could on this account not err, nor that everything they said would have to be correct. Great weakness and ignorance may be found to exist even in the most eminent people and hence we cannot judge concerning doctrines and matters of faith on the basis of personal holiness, for all this can fail. But here you come to the Word of God which is sure and infallible, where you shall certainly find Christ and the Holy Spirit, and can be and remain firmly fortified against sin, death, and the devil.
Source: Complete Sermons of Martin Luther (Volume 1) [Michigan: Baker Books, 2000] (First Sunday After Epiphany, Volume 1.2, pages 31-53). The above numbers #32, #38, #39, & #46 refer to the points in the sermon. The entire sermon can be found here:
http://www.lutherdansk.dk/Web-Fastepostillen%20AM/The%20Church%20Postil.htm