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 Polish delegation included Primate Mikołaj Trąba and Paweł Włodkowic, a scholar from the Cracow Academy. Włodkowic came to Constance with the doctrine of just war he had prepared in Cracow. His first success was even introducing the Polish-Teutonic issues to the programme of the Council, which he achieved by presenting the conflict with the Order in the context of the argument over the conditions of the just war. As long as the Council could evade the topic of territorial disputes, the question of justice and faith fell within the competence of the clergy.

In his reasoning, Włodkowic relied on the canonical law and natural law, and the theses he presented combined theology and politics. Neither the Teutonic Order nor anyone else has the right to invade pagan people under the pretext of conversion. The world was created for everyone and thus pagans have the same right to own lands as Christians. No ruler can invade a non-Christian land under the excuse of conversion and thus the Teutonic Knights should not have done that either.

Włodkowic’s appeal was not only one of the greatest achievements of Poland in their political fight against the Teutonic Order but also a huge contribution to the Christian theory of just war.

Dating to the early period of Polish statehood, this document – known as Dagome Iudex after its opening words – was written down circa 991. It is known only from an abstract compiled in Rome in the late 11th century. By means of this deed, Mieszko, the first historical ruler of Poland, together with his wife Oda and their two sons, entrusts his realm, which was centred on Gniezno, to the safekeeping of the Apostolic See.

Historians have made various conjectures about the intention of this document. Some argue that papal custody was meant to protect the independence of the Polish Church against the claims of the archbishops of Magdeburg; others argue that the point was to ensure the participation of Mieszko’s sons from his second marriage (with Oda) in the ruling of the state. This would explain why Mieszko’s eldest son Bolesław Chrobry (Boleslaus the Brave), whose mother was the Bohemian princess, Dobrava, was not mentioned. The document describes the borders of the territory ruled by Mieszko in the late years of his reign, which lasted from about 960 to his death in 992.