What prompted Ciszek's work in Russia as noted in the memoir, With God in Russia?

Walter became inspired by a plea from the Pope for young priests to consider entering a training program in Rome that would prepare them for missionary work in the godless Soviet Union. There were millions of Catholics in Russia who had no parish priests and were denied the Mass and the Confessional. Ciszek along with other young priests took on false identities in order to gain access to Russia. Had they tried to enter as priests, they would have been denied entrance.