Luther on Romans 10:1-2 (“Brothers, my heart’s desire and prayer to God for them [Israelites] is that they may be saved. For I bear them witness that they have a zeal for God, but not according to knowledge.”):
This is a prodigious statement, because it is the essential and sole opponent of faith, it resists obedience, it makes men stiff-necked and incorrigible, as is obvious in the case of heretics and schismatics. As if it were impossible for them to make a mistake, they stand firm and obstinate in their good intention, staking their entire salvation on the fact that they are carrying out their pious intention out of a zeal for God. Scripture very properly calls these men depraved or crooked in heart and corrupt in mind, even though they are not corrupt in the flesh or in bodily vices, but they are corrupt in spiritual matters, spiritually stubborn in their own ideas and good intention. Against these men the blessed Virgin is speaking when she says: “He has scattered the proud in the imagination of their hearts” (Luke 1:51). Today we have in this classification such people as popes and princes who neglect their own duties in their zeal for God and their pious intent and carry on business which does not concern them. Included also is every person who leaves a lower and good task which he ought to do and follows after a greater good, that is, something which has the appearance of a greater good, with the devil deceiving him all the time. For this zeal is the mind and head of the old serpent1 and the cause of all discord and the source, the father of pride and stubbornness and unbelief, and, [appearing to act] for the sake of salvation, it is a hindrance to salvation.
From “Lectures on Romans” by Martin Luther (Vol. 25, p. 405, Luther's Works, Concordia Publishing House, St. Louis, 1972).