Casimir grew up in a world where his life was not his own. As a prince of Poland, the second son of King Casimir IV and Elizabeth of Austria, his life was scheduled to cement his father's authority and increase Poland's power.

Casimir realized from an early age that his life belonged to someone else, but to a much higher King than his father. Despite pressure, humiliation, and rejection, he stood by that loyalty through his whole life.

Born the third of thirteen children in 1461, Casimir was committed to God from childhood. Some of that commitment was the result of a tutor, John Dlugosz, whose holiness encouraged Casimir on his own journey.