Son of Mikołaj "the Black" Radziwiłł and Elżbieta Szydłowiecka. Raised as a Calvinist, he converted to Roman Catholicism and was known as a very pious person, which gained him the nickname Pobożny (Pious). He was also known for his scholarly interests - he collected books and could read in many languages. He avoided politics, but fought in the wars with Muscovy under Polish king Stefan Batory.