At present many profess to adhere to some form of Anabaptism, they will attempt to mix and match all forms of (in my opinion incompatible) forms of Protestant and Roman Catholic practices and teachings and contriving a plethora of fantastical names to differentiate their supposed unique form of Anabaptism. Yet probably the one common thing a person can claim about these Neo-anabaptists is that they make a profession of orthodox belief regarding the nature of God and Christ. That is they claim to hold to the Trinity. It’s also more than likely that they would say that this is one of the earmarks of authentic Christianity. While Nicea and Chalcedon may have been the measuring rod for Protestantism and Roman Catholicism but what about Anabaptism or rather looking back was it a precondition for identifying Anabaptists?
My answer is no based on the fact that (1):
The Anabaptists never attached the weight to creeds or confessions given to them by the remainder of Christendom; they were biblicists who produced a large number of confessions, not as instruments to which the laity or ministry subscribed ex anima, but as instructional tools for the indoctrination of their young people and as witnesses to their faith for distribution in society or as a means of better understanding between differing groups.
Yes it is true that they produced their own confessions but that is the main point. Their beliefs and definition of Christianity come about through their study of scripture. They did not let the activities of others during a problematic time in Church history establish the rule for who qualifies as a Christian and who does not. Take the Schleitheim Confession of 1527, it had very little to do with orthodoxy if at all. It’s focus was on the process of coming to faith, establishing oneself as a member of the Body of Christ and Kingdom living. If the first generation Swiss Brethren had ever thought differently regarding any of the seven articles of the confession they would have altered them. The confessions was not considered inspired of God, they were just an outline of their beliefs at that time in history.
(2) When attempting:
to understand 16th-century Anabaptist notions of God it is most important to note that what distinguished Anabaptism from its Reformation counterparts—the Catholic, Lutheran, Reformed and Anglican traditions—was the extent of its theological and sociological diversity. It was not one homogeneous mass but a collection of diverse movements spread throughout Europe, defined by local differences which affected each group’s theology. Consequently, one cannot assume that there was one Anabaptist doctrine of God. Here as in other theological doctrines there was a dynamic plurality of views, cross-fertilizing each other and undergoing evolution especially during the early period.
This point is evident by the content of Anabaptist writings, in some instances you will find statements that at the least on the surface appear Trinitarian in nature but then later in another document authored by another Anabaptist writer you might find something that appears to be less Trinitarian or does not even address God’s nature at all. One will not find a detailed nuanced theological treatise in the fashion of Chalcedon or Nicea on the part of the Anabaptists. Some had different priorities when writing and speaking on the subject of God. And if anything that was written resembled orthodoxy it was done for the sake of argument.
(3) The initial generation did not concern themselves with systematic doctrinal articulations. Their focus was more on the practical aspects of the Christian life and doctrine. The early material from Swiss Brethren does not touch on God’s nature at all because their concern was on the affects that a relationship with the biblical God should have on the believer’s life.
(4) Some full-fledged Anabaptists did not believe in the Trinity at all.
The Polish Brethren
The Polish Brethren (a.k.a. the Minor Church) rejected the Trinity or in some instances Trinitarian language. Yet they are still Anabaptists in the truest sense. Thomas N. Finger explains that in the mid-late 16th century:
Although an enduring church body from each of Anabaptism’s original branches has attained distinctive form by now, believers’ baptism still spreads eastward. Eventually, the groups who are now adopting this rite will cease practicing it or die out altogether. Yet various threads connect these groups to other strands of our story. The Polish Brethren, who will endure for about a century, constitute a branch of historic Anabaptism.
George H. Williams corroborates this by stating “the Polish-speaking Anabaptism emergent within the context of the Antitrinitarian Minor Church of Poland and Lithuania was, both by analogy and by genetic succession, a regional variant of the Radical Reformation which swept over Central Europe in the sixteenth century.” The reason for the Polish Brethren’s questioning of orthodoxy was for the very thing people accused them of in the past and reason why contemporary individuals will not acknowledge the Polish Brethren as genuine Anabaptists or acknowledge them as being a part of their spiritual lineage. The Polish Brethren in the same fashion as those that proceeded them desired an ekklesia that was purely biblical in nature. They desired to do away with language that did not originate in the Bible and to not force “belief in anything beyond the minimum contained in Holy Scripture and the Apostles’ Creed, generally acknowledged for centuries, the Church removes the possibility of the propagation of heresy.”
Qualification as Authentic Anabaptists
According to C. Arnold Snyder “the Anabaptist movement’ included all the ‘adult baptizers’ of the sixteenth century.” Thus in his estimation the group had to be engage in adult baptism which during the 16th century was a unique and dangerous practice. Not too many would even fathom participating so the Anabaptists could stand out in this area.
Before moving forward it must be noted that there have been attempts at presenting an outline of specific doctrines that all Anabaptists had in common because of the emphasis and recurrence of certain themes in their writings but orthodox Trinitarianism could hardly be considered one of them. Articulating orthodoxy was not on their list of priorities as mentioned. So now the question is does the Polish Brethren meet the qualifications?
The Polish Brethren that made up “the historic core of the later anti-Trinitarian Socinians, held to adult baptism”. Not only that the “Polish Anabaptists, at the beginning at any rate, accepted nonresistance, too, as part of their religion. They rejected war and the magistracy as unchristian functions, just as the Swiss Brethren and the German and Dutch Mennonites did.” An item of note is that the Polish Brethren held to nonresistance to such a degree that they “produced perhaps the most interesting writings on nonresistance that have come down to us from the sixteenth century.”
When it relates to orthodoxy Anabaptists did not always blend with it, there has always been beliefs in play that many would call aberrant. A more notable example is the “celestial flesh” or “heavenly flesh” Christology of the Melchiorite Anabaptists and later the Mennonites. Much effort has been put forth to distance the Anabaptists from the Polish Brethren because of their beliefs and how Protestants and Roman Catholics would react to them. But Anabaptism was never about pleasing those bodies, it was about seeking truth found in God’s Word the Bible and applying what is learned to please God. To the Anabaptists no ecclesiastical authority could determine for them whether they was in the will of God. Therefore orthodoxy was never a determining factor when defining Anabaptism from a historical perspective.
 Christian Neff et al., Global Anabaptist Mennonite Encyclopedia Online (Harrisonburg, Virginia, and Waterloo, Ontario: Herald Press, 1989), s.v. “Confessions, Doctrinal,” accessed June 24, 2014, http://gameo.org/index.php?title=Confessions,_Doctrinal#1955_Article.
 A. James Reimer, Global Anabaptist Mennonite Encyclopedia Online (Harrisonburg, Virginia, and Waterloo, Ontario: Herald Press, 1989), s.v. “God (Trinity), Doctrine Of.,” accessed June 24, 2014, http://gameo.org/index.php?title=God_(Trinity),_Doctrine_of#Sixteenth-Century_Anabaptists.
 Thomas N. Finger, A Contemporary Anabaptist Theology: Biblical, Historical, Constructive (Downers Grove, Ill.: InterVarsity Press, 2004), 41.
 George H. Williams, “Anabaptism and Spiritualism in the Kingdom of Poland and the Grand Duchy of Lithuania: An Obscure Phase in the Pre-History of Socininianism”, in Ludwik Chmaj, ed., Studia nadarianizmem (Warsaw: Panstwowe Wydawnictwo Naukowe, 1959), 221.
 Stanislas Kot, Socinianism in Poland: The Social and Political Ideas of the Polish Antitrinitarians in the Sixteenth and Seventeenth Centuries, trans. Earl Morse Wilbur (Boston: Starr king press, 1957), 192-93.
 C. Arnold Snyder, Anabaptist History and Theology: An Introduction (Kitchener, Ont.: Pandora Press, 1995), 6.
 George Huntston Williams and Angel M. Mergal, eds., Spiritual and Anabaptist Writers, The Library of Christian Classics (1957; repr., Louisville: Westminster John Knox Press, 2006), 20.
 Peter Brock, ed. “A Polish Anabaptist Against War: The Question of Conscientious Objection in Marcin Czechowic’s Christian Dialogues of 1575”, Mennonite Quarterly Review 52, no. 4 (1978): 279.
 Ibid., 280.