The Minor Reformed Church of Poland, better known today as the Polish Brethren, was started on January 22, 1556, when Piotr Giezek, a Polish student (also known as Piotr z Goniadza or Peter Gonesius), attacked the doctrine of the Trinity during the general synod of the Reformed (Calvinist) churches of Poland held in the town of Secemin. A theological debate called by the Polish king himself in 1565 did not succeed in bringing both Protestant factions together again. Finally, the faction that had supported Giezek's arguments broke all ties with the Calvinists and organized their own synod in the town of Brzeziny on June 10, 1565. Originally, the Minor Church followed a non-trinitarian doctrine inspired by the writings of Michael Servetus. Later on, Socinianism, named for Italian theologian Laelius Socinus, became its main theological approach.
The Minor Church ended in Poland with the expulsion of Arians from Poland in 1658. The Brethren never participated in the Sandomierz Agreement between different Polish Protestants. They advocated the separation of church and state and taught the equality and brotherhood of all people; they opposed social privileges based on religious affiliation, and their adherents refused military service (they were known for carrying wooden swords instead of real almost obligatory szablas) and declined political office. They were against capital punishment, and did not believe in the traditional Christian doctrines of Hell or the Trinity.
Although never numerous, they had a significant impact on political thought in Poland. After being expelled from Poland, they emigrated to England, East Prussia and the Netherlands, where their works were widely published and influenced much of the thinking of later philosophers such as John Locke and Pierre Bayle.
Their main ideologues were Piotr z Goniadza (Gonesius), Grzegorz Paweł z Brzezin, although Johannes Crellius (originally from Germany), and Jan Ludwik Wolzogen (who came to Poland from Austria) were far better known outside Poland. Among the best known adherents of this fellowship are Mikołaj Sienicki, Jerzy Niemojewski, and writers and poets Zbigniew Morsztyn and Wacław Potocki.
Their biggest cultural center were Pińczów and Raków, site of the main Arian printing press and the university Akademia Rakowicka (Gymnasium Bonarum Artium) founded in 1602 and closed in 1638, which trained over 1000 students.
These men were exiled from Poland in 1658 after a series of 17th century wars known as The Deluge in which protestant Sweden invaded Poland, since they (as almost all non-Catholics) were commonly seen as Swedish collaborators. This expulsion is sometimes taken as the beginning of decline of famous Polish religious freedom, although the decline started earlier and ended later: the last non-Catholic deputy was removed from parliament in the beginning of the 18th century. Most of Polish Brethren moved to the Netherlands, where they greatly influenced European opinion, becoming precursors to Enlightenment. Through their connection to Enlightenment thinkers, their ideas also influenced the Founding Fathers of the United States.
In the Second Polish Republic, 1937, priest Karol Grycz-Śmiałowski recreated the Church of Polish Brethren in Kraków. In the People's Republic of Poland it was registered in 1967 as the Unity of Polish Brethren (Jednota Braci Polskich).
John Locke was preceded by a few decades by Samuel Przypkowski on tolerance and by Andrzej Wiszowaty on 'rational religion.' Isaac Newton had met Samuel Crell, son of Johannes Crellius, of the Spinowski family. Newton was well informed about the developments in Poland and collected many books from the Racovian Academy.
Englishman John Biddle had translated two works by said Przypkowski, as well as the Racovian Catechism and a work by Joachim Stegmann, a "Polish Brother" from Germany. Biddle's followers had very close relations with the Polish Socinian family of Crellius (aka Spinowski).
Subsequently, the Unitarian branch of Christianity was continued by, most notably, Joseph Priestley, who had emigrated to the United States and was a friend of both James Madison and Thomas Jefferson, who claimed to be a Unitarian and credited Priestley with having converted him to that faith. Notably, Priestley was very well informed on the earlier developments in Poland, especially by his mentions of Socinus and Szymon Budny (translator of Bible, author of many pamphlets against the Trinity).
Modern groups which look to the Polish Brethren include the Christadelphians and CoGGC. Although Christadelphians had since their origins in the 1840s always looked for historical precedents, particularly to Arius the group was unaware of closer precedents in Socinianism. This changed with a series of articles in the community magazine during the early seventies subsequently published.  The Polish arm of the Christadelphians use the name Bracia w Chrystusie in conscious echo of Socinian precedents. The Atlanta Bible College of the CoGGC also publish a Journal continuing research into the Polish Brethren and related groups. 
- ↑ See Hewett, Racovia, pp. 20-21.
- ↑ Hewett, p. 24.
- ↑ Snobelen, Stephen D. (1999). "Isaac Newton, heretic: the strategies of a Nicodemite" (PDF). British Journal for the History of Science 32: 381–419. doi:10.1017/S0007087499003751. http://www.isaac-newton.org/heretic.pdf.
- ↑ Eyre, Alan, The Protestors, Birmingham 1975
- ↑ Eyre, Alan, Brethren in Christ, Adelaide, 1983
- ↑ http://pl.wikipedia.org/wiki/Bracia_w_Chrystusie
- ↑ http://www.abc-coggc.org/jrad.html Journal for The Radical Reformation
- Phillip Hewett, Racovia: An Early Liberal Religious Community, Providence, Blackstone Editions, 2004.
- Joseph Kasparek, The Constitutions of Poland and of the United States: Kinships and Genealogy, Miami, FL, American Institute of Polish Culture, 1980.
- Earl Morse Wilbur, A History of Unitarianism: Socinianism and Its Antecedents, Harvard University Press, 1945.
- George Huntston Williams, The Polish Brethren : Documentation of the History and Thought of Unitarianism in the Polish-Lithuanian Commonwealth and in the Diaspora 1601–1685, Scholars Press, 1980, ISBN 0-89130-343-X
- Polish Socinians: FROM THE POLISH SOCINIANS TO THE AMERICAN CONSTITUTION, Marian Hillar, A Journal from the Radical Reformation, A Testimony to Biblical Unitarianism, Vol. 4, No. 3, pp. 22-57, 1994
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