At the same time appeared the tendency aiming at unification on the Catholic side. The Catholic Church was undergoing intensive reforms, connected with the Council of Trent, whose tone was set by Jesuits, who had significant influence on the Polish king and the overall course of the state affairs. Their most eminent representative, Piotr Skarga, penned a work devoted to the unification of churches entitled O rządzie i jedności Kościoła Bożego pod jednym pasterzem [on the administration and unity of God’s Church under one shepherd]. It was read by Sigismund III Vasa. Also fearing that Moscow would subordinate the Polish Orthodox Church to itself, the king became a proponent of the union of the Churches. At the same time Jesuits established a network of schools offering a high level of education, which became highly popular with the Orthodox nobility. Those schools built the positive image of the Catholic Church which in time resulted in the Orthodox nobility becoming more favorable to Rome.

Under those circumstances, the bishops who supported the union began negotiations with the king and Catholic hierarchs. In 1595 representatives of the Polish Orthodox clergy went to Rome, where they made a confession of faith before the Pope and accepted the union’s conditions, which stipulated for retaining the rites, calendar, and hierarchy, and also subordination to the Pope. The Council of Brest was soon called, where the act of union was signed, thus establishing the church known as the Greek Catholic or Uniate Church. A minority of the clergy did not join the union, remaining in the Orthodox Church, from then on called disuniates.

The signatories of the ceremonious document – affixed, as seen below, with numerous seals – regulated the most urgent matters. One of them is dealt with in points 3 and 4 – namely, the guarantee to uphold peace between the adherents of the diverse confessions then practised in the Rzeczpospolita (Commonwealth) The signatories mutually swore to oppose any and all attempts to initiate persecutions for reasons of religion. At the same time it was stressed that religious matters could not be used as a pretext for undermining the social order.

Of surpassing significance for the origins of the provisions upholding religious freedom was the fact that among the leading candidates to the vacant throne was the son of the French king, Henry (de Valois) who had been charged with having directly participated in the slaughter of French Protestants (Huguenots) during the St. Bartholomew’s Day Massacre of August 23-24, 1572 in Paris. In accordance with the stipulations of the Warsaw Confederation, when Henry indeed was elected to the throne of the Rzeczpospolita (in May, 1573), the Polish delegation that traveled to France demanded of the king-elect that he swear an oath to uphold the principles of the Warsaw Confederation. Henry did so in Notre Dame Cathedral in September that year.

The fact that freedom of religion was made part of the Henrician Articles attests that the provisions of the Warsaw Confederation were to remain binding indefinitely, rather than only during the interregnum and Henry’s reign.