The Lithuanian nobles obliged Chodkevičius in the Brasta sejm to implement the union between Livonia and the Grand Duchy of Lithuania, as well as to join it with Riga and Swedish-occupied Tallinn.

[1] During the preparation of the documents of the Union of Lublin (1569), he persistently defended the state interests of the Grand Duchy of Lithuania and led its delegation.

[citation needed] The Lithuanian delegation to the meetings preparing the Act of Union between Poland and Lithuania was led by Jan, who insisted in a long impassioned speech on the equality and independence of the two nations.

[2] In practice the Union of Lublin in 1569 made sure that the Grand Duchy of Lithuania retained its own form of government and separate laws until the end of the joint state in 1795.

[citation needed] He supported the candidacies of the French Henri de Valois, the Russian Ivan IV the Terrible, and the Hungarian Stephen Báthory to the throne of the ruler of the Republic of the Two Nations.

[3] He married the Calvinist Krystyna Zborowska before 1559 in Kraków, daughter of Marcin Zborowski, castellan of Krakow, and Anna Konarska.

Jan Hieronim married Krystyna Zborowska h. Jastrzębiec (c. 1540–1588), daughter of Marcin Zborowski h. Jastrzębiec and Anna Konarska h. Abdank, the daughter of Stanisław Konarski h. Abdank and Zofia Lanckorońska h. Zadora, in 1559, and had seven children:[4] Danuta Bogdan, Students of the Republic at the University of Königsberg, in Królewice and Poland, Olsztyn 1993, p 82.

Cracow : Polish Academy of Learning – Main Ingredients in bookstores Gebethner and Wolff, 1937, pp. 361–363.