Luther vigorously critiqued many of his fellow Protestants.
Protestant founder Martin Luther's radical change of the rule of faith from an infallible Bible and Church and tradition to private judgment and sola Scriptura (Scripture as the only infallible authority) and comments about plowboys being able to interpret Scripture without the checks and balances of that Church and tradition, naturally led to excesses of individuality and sectarianism.
People reasoned (consciously or not) that since Luther felt free to break away from Catholicism and gave the example of an ongoing smear campaign of propaganda and calumny against the existing Church, that there was little reason why they could not reject both the Catholic Church and him.
In other words, he was naive to think that he could unleash an entirely new principle, yet expect that no one besides him would utilize it, in precisely the way that he had. Hence, Carlstadt and the Anabaptists and Zwinglians and Calvinists and other groups arose, to his great dismay. The truth (whatever it was) was not self-evidently clear to all from Scripture alone. He failed to see any connection whatever between his teachings on authority, and what ensued. Yet the connection is almost self-evidently obvious.
The Catholic Encyclopedia (1913), in its article on Luther, made the same general point:
Luther by the creation of his “universal priesthood of all Christians”, by delegating the authority “to judge all doctrines” to the “Christian assembly or congregation”, by empowering it to appoint or dismiss teacher or preacher, sought the overthrow of the old Catholic order. It did not strike him, that to establish a new Church, to ground an ecclesiastical organization on so precarious and volatile a basis, was in its very nature impossible. The seeds of inevitable anarchy lay dormant in such principles.
Luther vigorously critiqued many fellow Protestants: the Anabaptists (who “won't have baptism”) and Zwinglians and proto-Calvinists, or those groups known as “sacramentarians” (those who “deny "the efficacy of the Lord's supper”).
Thus, Luther condemned beliefs that are hardly distinguishable from present-day Baptists, or anyone who holds to adult baptism, non-regenerative baptism, or who denies the Real Presence in the Eucharist. This would include the vast majority of Protestant evangelicals and Calvinists. Luther would almost certainly regard them all as damned.
He regarded fellow Protestant “reformers” like Zwingli and Martin Bucer and Oecolampadius as damned. Thus, he regarded Zwingli's 1531 slaying on the battlefield as evidence of God's judgment for his having forsaken the Christian faith.
The following excerpts of Luther were drawn from his Letter to the Christians of Antwerp (1525); found on pp. 91-92 in Jules Michelet, The Life of Luther Gathered From His Own Writings, (translated by G. H. Smith, London: Whittaker & Co., from the original 1835 work):
The devil seeing that this sort of disturbance could not last, has devised a new one; and begins to rage in his members, I mean in the ungodly, through whom he makes his way in all sorts of chimerical follies and extravagant doctrines. This won't have baptism, that denies the efficacy of the Lord's supper; a third, puts a world between this and the last judgment; others teach that Jesus Christ is not God; some say this, others that; and there are almost as many sects and beliefs as there are heads.
I must cite one instance, by way of exemplification, for I have plenty to do with these sort of spirits. There is not one of them that does think himself more learned than Luther; they all try to win their spurs against me; and would to heaven that they were all such as they think themselves, and that I were nothing! The one of whom I speak assured me, amongst other things, that lie was sent to me by the God of heaven and earth, and talked most magnificently, but the clown peeped through all. At last, he ordered me to read the books of Moses. . . .
I have plenty to do in the course of the year with these poor people: the devil could not have found a better pretext for tormenting me. As yet the world had been full of those clamorous spirits without bodies, who oppressed the souls of men; now they have bodies, and give themselves out for living angels . . .
When the pope reigned we heard nothing of these troubles. The strong one (the devil) was in peace in his fortress; but now that a stronger one than he is come, and prevails against him and drives him out, as the Gospel says, he storms and comes forth with noise and fury.
Dear friends, one of these spirits of disorder has come amongst you in flesh and blood; he would lead you astray with the inventions of his pride: beware of him.
First, he tells you that all men have the Holy Ghost. Secondly, that the Holy Ghost is nothing more than our reason and our understanding. Thirdly, that all men have faith. Fourthly, that there is no hell, that at least the flesh only will be damned. Fifthly, that all souls will enjoy eternal life. Sixthly, that nature itself teaches us to do to our neighbour what we would he should do to us ; this he calls faith. Seventhly, that the law is not violated by concupiscence, so long as we are not consenting to the pleasure. Eighthly, that he that has not the Holy Ghost, is also without sin, for he is destitute of reason.
All these are audacious propositions, vain imaginations; if we except the seventh, the others are not worthy of reply. . . .
It is sufficient for us to know that God wills no sin. As to his sufferance of sin, we ought not to approach the question. The servant is not to know his master's secrets, simply his master's orders: how much less should a poor creature attempt to scrutinize or sound the mysteries and the majesty of the Creator? . . .