The Polish Brethren were members of the Minor Reformed Church of Poland, a Nontrinitarian

Nontrinitarianism

Nontrinitarianism includes all Christian belief systems that disagree with the doctrine of the Trinity, namely, the teaching that God is three distinct hypostases and yet co-eternal, co-equal, and indivisibly united in one essence or ousia...

 Protestant church that existed in Poland

Poland

Poland , officially the Republic of Poland , is a country in Central Europe bordered by Germany to the west; the Czech Republic and Slovakia to the south; Ukraine, Belarus and Lithuania to the east; and the Baltic Sea and Kaliningrad Oblast, a Russian exclave, to the north...

 from 1565 to 1658. By those on the outside, they were called "Arians

Arianism

Arianism is the theological teaching attributed to Arius , a Christian presbyter from Alexandria, Egypt, concerning the relationship of the entities of the Trinity and the precise nature of the Son of God as being a subordinate entity to God the Father...

" or "Socinians

Socinianism

Socinianism is a system of Christian doctrine named for Fausto Sozzini , which was developed among the Polish Brethren in the Minor Reformed Church of Poland during the 15th and 16th centuries and embraced also by the Unitarian Church of Transylvania during the same period...

" (Polish: arianie, socynianie), but themselves preferred simply to be called "Brethren" or "Christians," and, after their expulsion from Poland, "Unitarians

Unitarianism

Unitarianism is a Christian theological movement, named for its understanding of God as one person, in direct contrast to Trinitarianism which defines God as three persons coexisting consubstantially as one in being....

".
The Ecclesia Minor or Minor Reformed Church of Poland, better known today as the Polish Brethren, was started on January 22, 1556, when Piotr of Goniądz (Peter Gonesius), a Polish student spoke out against the doctrine of the Trinity during the general synod of the Reformed (Calvinist) churches of Poland held in the village of Secemin

Secemin

Secemin is a village in Włoszczowa County, Świętokrzyskie Voivodeship, in south-central Poland. It is the seat of the gmina called Gmina Secemin. It lies approximately south-west of Włoszczowa and west of the regional capital Kielce.The village has a population of 1,600.-References:...

.
A theological debate called by the Polish king Sigismund II Augustus

Sigismund II Augustus

Sigismund II Augustus I was King of Poland and Grand Duke of Lithuania, the only son of Sigismund I the Old, whom Sigismund II succeeded in 1548...

 himself in 1565 did not succeed in bringing both Protestant factions together again.