 Two years of novitiate in Vilnius (1754–56) were followed by a bout of teaching at Kražiai College (1756–58) and theology studies in Pinsk (1759–63) where he was ordained priest in 1762.After being a missionary in several places, Minsk, Nesvizh, and Slutsk, Kareu arrived in 1768 in Polatsk where he studied architecture under the guidance of Gabriel Lenkiewicz, along with teaching philosophy and mathematics at the Jesuit College in Polatsk (1769–72).Being close to Stanislaus Czerniewicz and Lenkiewicz, he helped them in their efforts to keep the Society of Jesus going in Russia.