From Dr. Luther's 1535 Lectures on Galatians:
"‘But if I, brethren, still preach circumcision, why am I still persecuted? In that case the stumbling block of the cross has been removed’ (Gal. 5:11).
"Here Paul wants to show that it would be an absurdity and a disgrace if the stumbling block of the cross were to end. He speaks the same way in First Corinthians (1:17): ‘Christ sent me to preach the Gospel, not with eloquent wisdom, lest the cross of Christ be emptied of its power.’ It is as though he were saying: ‘I would not be willing to remove the stumbling block and the cross of Christ.’ Here someone may say: ‘Christians must be quite insane if they expose themselves to dangers voluntarily. For all they accomplish with their preaching is to gain for themselves the anger and hatred of the world and to create stumbling blocks. And that, as the saying goes, is laboring in vain and simply looking for trouble.’ ‘This fact,’ says Paul, ‘does not offend or bother us at all; it only makes us courageous and optimistic about the success and growth of the church, which flourishes and grows under persecution.’ For Christ, the Head and the Bridegroom of the Church, must ‘rule in the midst of His foes’ (Ps. 110:2). On the other hand, when the cross and the raging of tyrants and heretics have been removed, and the stumbling blocks have come to an end, and when the devil ‘guards his own palace, and his goods are in peace’ (Luke 11:21), this is a sure sign that the pure teaching of the Word has been taken away.
"Bernard had this in mind when he said that the church is in the best position when it is under pressure on every side from the power and craft of Satan, and that it is in the worst position when it is most at peace. By a fine use of catachresis he cites this statement from the canticle of Hezekiah: ‘Lo, in peace was my greatest bitterness’ (Is. 38:17) and put it into the mouth of the church when it is living in security and peace. Therefore Paul regards it as a sure sign that what is being preached is not the Gospel if the preaching goes on without its peace being disturbed. On the other hand, the world regards it as a sure sign that the Gospel is a heretical and seditious doctrine when it sees that the preaching of the Gospel is followed by great upheavals, disturbances, offenses, sects, etc. Thus God wears the mask of the devil, and the devil wears the mask of God; God wants to be recognized under the mask of the devil, and He wants the devil to be condemned under the mask of God.
"The term ‘stumbling block of the cross’ may be understood either actively or passively. The cross immediately follows the teaching of the Word, in accordance with the statement of Psalm 116 (v. 10): ‘I believed; therefore I have spoken. But I am greatly afflicted.’ Now the cross of Christians is ignominious and merciless persecution; therefore it is a great stumbling block. To begin with, they suffer as though they were the vile scoundrels. The Prophet Isaiah predicted this about Christ Himself: ‘He was numbered with the transgressors’ (Is. 53:12). In addition, the punishments of thieves and criminals are commuted, and people are touched by pity toward them, so that there is no stumbling block connected with the punishment. But because the world regards Christians as dangerous men, it believes that no punishment that can be inflicted on them is severe enough. Nor is it touched by any pity toward them, but it imposes the most shameful kind of death on them. By this means it seeks to gain a dual advantage: first, it offers service to God by killing them (John 16:2); and secondly, it hopes to reestablish public peace by getting rid of these nuisances. Thus the cross and death of the godly are filled with stumbling blocks. ‘Do not let that bother you,’ says Paul, ‘the inhuman treatment and the continuance of the cross and of the stumbling block, but rather let it encourage you; for as long as these things continue, the Christian cause is doing very well.’
"Christ also comforts His followers in this way: ‘Blessed are you when men revile you and persecute you and utter all kinds of evil against you falsely on My account. Rejoice and be glad, for your reward is great in heaven; for so men persecuted the prophets who were before you’ (Matt. 5:11–12). The church will not permit this joy to be taken away from it. Therefore I would not want the pope, the bishops, the princes, and the fanatical spirits to be in accord with us; for such accord would be a sure sign that we had lost the true doctrine. In short, the church must suffer persecution because it teaches the Gospel purely. The Gospel proclaims the mercy and the glory of God; it discloses the wickedness and the wiles of the devil, portraying him in his true colors and taking away his mask of divine majesty, by which he has made an impression on the whole world. That is, it shows that all the forms of worship, religious ways of life, and monastic orders invented by men, as well as the traditions about celibacy, special foods, etc., by which men think they can gain the forgiveness of sins and justification, are all ungodly things and ‘doctrines of demons’ (1 Tim. 4:1). Thus there is nothing that vexes the devil more than the proclamation of the Gospel; for this takes away from him the mask of God and shows him up for what he is, not God but the devil. Therefore it is unavoidable that when the Gospel flourishes, the stumbling block of the cross will follow; otherwise it is sure that the devil has not really been attacked but has only been gently caressed. If he is really attacked, he does not remain quiet but begins to raise a terrible disturbance and to create havoc everywhere.
"If Christians want to keep the Word, they must not be offended or frightened when they see the devil breaking his reins and running wild, or the whole world in tumult, or tyrants in rage, or sects arising. But they should know for a certainty that these are signs, not of terror but of joy, as Christ interpreted them when He said: ‘Rejoice and be glad’ (Matt. 5:12). Therefore may the stumbling block of the cross never be taken away, which is what would happen if we were to preach what the ruler of this world (John 14:30) and his members would like to hear, namely, the righteousness of works; then we would have the devil friendly to us, the world on our side, and the pope and the princes kindly disposed toward us. But because we illumine the blessings and the glory of Christ, they persecute us and rob us of our goods and our very lives."
(Luther’s Works, Volume 27, CPH 1963)