PETRIKAU, pe'tri-kau', SYNODS OF. Four Polish synods held at Petrikau (75 m. s.w. of Warsaw), Russian Poland, in 1551, 1555, 1562, and 1565. The Reformation early found welcome in Poland, especially in Posen and Cracow; and the first Protestant teachers were exclusively Lutheran. Calvinism was introduced during the reign of Sigismund August II. (1548-72), who stood in close relations to Calvin, and at the same time the Bohemian Brethren expelled from their own country took refuge in large numbers in Great Poland, especially in Posen. At the Synod of Kozminek in 1555 they united with the Calvinists, though the Roman Catholics, under the leadership of Stanislaus Hosius, bishop of Culm and Ermeland, did all in their power to obstruct the extension of the Protestant movement.
At the first Synod of Petrikau in 1551, a Roman Catholic confession of faith was drawn up, expressly intended to answer the principles of the Augsburg Confession, and severe measures were taken against converts to the new teachings. The king and the nobility, however, strongly favored the Protestant party, and the former added his voice to the demand made by the second Synod of Petrikau (1555) that a national council be convened to settle the religious controversies. Sigismund also sent representatives to the pope, requiring the administration of the chalice, the celebration of mass in the vernacular, the abolition of clerical celibacy, and the abandonment of annates. The pope, however, refused to accede to these demands, and sent a nuncio, Bishop Lipomani of Verona, to Poland to repress the Protestant movement. He entirely failed, but the success of the Polish reformers was rendered impossible by their own divisions, as became clear at the third synod, held at Petrikau in 1562. There were constant difficulties between the Lutheran and Reformed parties, and the situation was made still more complicated by the appearance of a Polish antitrinitarian movement. All attempts to secure harmony failed, and the antitrinitarians were formally excluded from fellowship with Protestants at the fourth synod of Petrikau, held in 1565, though neither this nor a royal command banishing all Italian antitrinitarians (1654) was carried out.
In the same year, at a diet convened at Petrikau, the antitrinitarian leaders secured the holding of a disputation with their opponents, though the Lutherans held aloof, and only the Reformed and the Bohemian Brethren accepted. At this disputation Gregor Pauli, a Cracow preacher and the leader of the antitrinitarians, alleged the impossibility of reconciling the Catholic creeds concerning the Persons of the Trinity with the teaching of the Scriptures; while the trinitarians insisted on the historic agreement between the Scriptures and the teaching of the whole Church. After fourteen days of debate the two parties were farther apart than ever. The antitrinitarian representatives, moreover, disagreed among themselves, some denying the preexistence of Christ and the personality of the Holy Spirit, others accepting the preexistence of Christ and the reality of the Holy Spirit, and yet others assuming three Persons in the Trinity, but ascribing different values to them. The final outcome of the matter was the exclusion of the antitrinitarians from the Reformed Church, so that henceforth they constituted a separate communion.