This study is based on an analysis of Moravian Calvinist Jan Opsimathes’s book of friends (cca. 1568–after 1620), housed today at the British Library in London at the shelf mark Eg. 1220. The album contains roughly 590 entries spanning the years 1598–1620 and represents a valuable source for the study of the contacts between the Czech Lands and intellectual and political Calvinism across Europe. At the core of the study is an analysis of the album’s entries authored by individuals originating from Greater Poland, Lesser Poland, Lithuania, Livonia and other lands falling within the Polish king’s sphere of influence. The author was able to find a total of 21 such individuals. They encountered Opsimathes at Swiss, Dutch and German academies and universities, during his travels in France and, exceptionally, in Prague and Moravia. Although most of them were of noble origins, they differed considerably with regard to aristocratic descent, economic status, ranks attained and political sway. At one end was a group of lesser nobles and intellectuals; in the middle a group of relatively wealthy regional aristocrats; and at the other end a group of Polish, Lithuanian and Livonian notables (Andrzej and Rafał Leszczyński, Jan Radzimiński, Mikołaj Abramowicz and Magnus Ernest Denhoff ). Their common unifying trait was their close adhesion to the Calvinist confession and, in certain instances, the Polish branch of the Unity of the Brethren. The study also offers new information on the life of Jan Opsimathes based on freshly unearthed sources.