ENThe article focuses on Mikołaj Grzegorz Kuczewski (1713–1768), court chaplain of Michał Kazimierz Radziwiłł „Rybeńko”, grand hetman of Lithuania and Vilnius voivode, and preceptor of his siblings Janusz Tadeusz (1734–1750) and Karol Stanisław „Panie Kochanku” (1734–1790). For more than 25 years, besides pastoral ministry and education, the Jesuit father fulfilled various tasks, such as cataloguing the Radziwiłł library, publishing religious and courtly literature, and assisting his pupils during their first public performances. Kuczewski’s high status at the Nieśwież court made him an influential ally to the factions competing for the magnate’s protection. The Jesuit father, however, did not take advantage of this fact and kept a low profile. The author exploits many sources, including records from the Jesuit archives, memoirs, the press, and the Radziwiłłs’ private correspondence. Among the latter, Kuczewski’s letters, mainly addressed to “Rybeńko” and his son, allow a deep insight into the activities of this court chaplain. The article portrays Kuczewski as a religious client of the Radziwiłłs. By avoiding the stereotypical image of a client as the executor of his patron’s orders, the author also highlights Kuczewski’s contribution to the hetman’s agenda. Keywords: history of education; Society of Jesus; Grand Duchy of Lithuania; 18th century; Radziwiłł.