Menno, and Anabaptists generally, did not accept Luther's forensic doctrine of justification by faith alone because they saw it as an impediment to the truth doctrine of a 'lively' faith which issues in holy living.
Phil Johnson, executive director of Grace to You, John MacArthur's radio program, wrote online at his Spurgeon website:
But in [rejecting forensic justification] they [the Anabaptists] undermined the very foundation of the biblical doctrine of justification. They left people to try to devise a righteousness of their own to those who believe (cf. Phil 3:9; Rom 4:5-6).
Perhaps wanting to increase the validity of his original statement, in a Twitter on January 23, 2010, Johnson wrote:
Show me one anabaptist confession or treatise on justification that is sound & thorough enough to refute the Council of Trent.
That statement by Johnson reminded me of an essay written in 1987 by the late Richard Weeks, Lessons Learned from Baptist History, in which he wrote his third lesson: "Church History was written primarily by those who hated New Testament Christianity." After introducing that lesson, he said:
[T]hey were continually being persecuted, often fleeing, meeting in secret, and afraid to make written records that could fall into the hands of their enemies and be thus material for their condemnation. Most of their leaders were a "lay" ministry with little time to write because of making a living in a secular occupation, a circumstance diametrically different from the entrenched Roman clergy and later the Lutheran, Reformed, and Anglican full-time ordained literary-trained ecclesiastics.
Weeks continued about the kind of material that was published about these New Testament Christians:
Anything written in a derogatory expression that would help to counteract [them] was valid. . . . While the New Testament Christians had neither time or opportunity to write church history, their enemies did, and they wrote to express deep animosity of the constant threat pure New Testament truth presented to them.
Thank God, in the last one hundred years historian-scholars of most all denominations have by open historical research come to agree that the Biblical Anabaptists were terribly and falsely maligned by their opponents and that the actual truth is that they were a most noble, peaceable, harmless, exemplary people, a wonderful credit to Jesus Christ and remarkably different from the corrupt entrenched crowd that despised them.
According to William Estep in his excellent, The Anabaptist Story (1975), Anabaptists could not be identified by any unifying creed, primarily because of their insistence on the primacy of Scripture. He shows how that Menno Simon was to have said that the church fathers themselves should be trusted only if supported by God's Word (pp. 130, 133) . Some men that were called "Anabaptist" surely made the Scripture secondary to the internal voice of the Spirit, leading to certain tragedies and a smearing of those who depended on the Bible for authority. Because the Anabaptists, differing from the Protestants and Catholics, did not depend on coercion and would not enforce unity by the sword, neither could they rely on those methods to unify some movement.
Despite the fact that it was very difficult for the Anabaptists and other Baptist forefathers to publish and distribute written materials, we still have evidence that what George and Johnson say is wrong. Anabaptist Michael Sattler, the author of the Schleitheim Confession, wrote in "Concerning the Satisfaction of Christ":
Paul says to the Romans in the third chapter that they are all together sinners and come short of the glory which God should have from them, yet apart from merit, they shall be justified by his grace through redemption which Christ accomplished.
In 1956 Leonard Verduin translated the works of Anabaptist Menno Simon (1496-1561) and this was recorded in The Complete Writings of Menno Simon (p. 79):
Through the merits of Thy blood we receive the remission of our sins according to the riches of Thy grace.
In 1992 Cornelius Dyck, William Keeney, and Alvin Beachy translated and edited The Writings of Dirk Philips, another Anabaptist (1504-1568), and he wrote (p. 69):
He has justified us out of grace without merit through the redemption that has taken place in him, Rom. 3:21-25. He has set before us the selfsame one as a mercy seat through faith in his blood, Eph. 1:5-8, and has included all those under sin in order that he alone may be justified and in turn justify all who have faith in Jesus Christ, Rom. 3:19-26.
That all sounds like forensic justification. So where is the disconnect for George and Johnson? I think it is a couple of issues. First, a large majority of reformed historians were not trustworthy in their representation of the Anabaptists. Instead of relying on exactly what the Anabaptists themselves wrote and said, these men count on what so-and-so Mr. Reformed Historian has said that the Anabaptists have said even if they didn't say it. Either that or they broad brush all the Anabaptists based on the beliefs of some of the less orthodox who were also called Anabaptist. The magisterial reformers, the state church guys, expected submission and obedience from subjects of the state. They didn't like the Anabaptists because they wouldn't submit in state-approved doctrine, so they didn't mind slandering them or misrepresenting them.
Second, the Anabaptists had a different emphasis in salvation than the magisterial reformers. Anabaptists put an emphasis on the new birth rather than the justification, stressing that the imputed righteousness would show up in a life of surrender. The Anabaptists saw a lot of unconverted-like behavior in the lives of the reformational leadership, let alone the rank and file. The reformed view of justification tied into their practice of infant sprinkling as well. Many of the reformation period were members of the state church and had not experienced conversion. For a taste of what the Anabaptists experienced, one need only look at what Baptists also faced in the American colonies from the state church of the Massachusetts Bay Colony. When George Whitefield came to the colonies to preach in the open air, most of the church members in Massachusetts were not saved.
Third, and this is my opinion. The reformed and/or the Calvinists want to bring the non-reformed, non-Calvinist Anabaptist into disrepute. They do not want a non-Calvinist soteriology to look historic. They want the true Baptists to be the English separatists that came out of the reformation. You can see some of the venom coming from Johnson in a Twitter that he made shortly after the one he wrote above: "I've had a standing challenge to the Trail-of-Blood revisionists for more than a decade, and it has gone unanswered." There are several views of Baptist history, one being called the "Trail-of-Blood" position. Not everyone who believes in the succession of the New Testament church, based upon scriptural presuppositions, goes with the "Trail-of-Blood" per se, even if they do believe in a persecuted church. There is also the spiritual kinship position that was advocated by Thomas Armitage in his History of Baptists. Of course, Johnson also calls them "revisionists," which is parallel with calling them liars. Do you think that someone is revising history (lying) that makes a quotation directly from the Anabaptist's actual writings with page numbers included? Johnson needs to take a step back from his accusation.
Another important point to consider comes from a paragraph by Guy Hershberger in his book, The Recovery of the Anabaptist Vision (p. 65):
In 1527 Michael Sattler, the Anabaptist from the Black Forest, drew up a confession of faith in seven points. It was adopted by a secret synod of South German and Swiss Anabaptists at Schleitheim near Schauffhausen, as a binding Rule of Faith, and has become known as the Schleitheim Confession. The Schleithem articles are Anabaptism's oldest confessional document. It is first of all striking that these articles say nothing about God, Jesus Christ, and justification by faith. The central truths of the Christian faith are not mentioned. Why? Because the men who adopted this confession were in agreement with Luther and Zwingli concerning all of these central truths. Zwingli himself emphasized repeatedly that nothing involving belief in God, Christ, and grace separated the Anabaptists from him. The Schleithem Confession deals only with those points in which Anabaptism and the Reformation differ. That is the reason for the absence of the fundamental Christian truths. The seven articles deal with the church and the state, not with the center of the Christian faith. In faith in Jesus Christ as sole and sufficient Redeemer they were of one mind with the Reformers.
If someone were to read the Schleitheim Confession and conclude that Sattler and the Anabaptists did not believe in justification by faith in Christ alone, he would also need to conclude that they did not believe in God or the Deity of Christ either. It is convenient to cherry-pick the "missing" justification by faith. It does show, however, a fundamental misunderstanding of the Schleitheim Confession. They were advocating their belief in justification by faith through its absence from that short Confession. The Schleitheim Confession recorded only the differences.
So did the Anabaptists believe in justification by faith alone? Did they oppose the Council of Trent? Of course they did.