Hearing of the great sanctity of Dominic of Osma, who had recently been attributed the miracle of resuscitating the nephew of Cardinal Stefano di Fossa Nova who had been killed in a fall from his horse,[1][2] Ceslaus, together with Hyacinth, sought admission into the Order of Friars Preachers.

In 1219 Pope Honorius III invited Dominic and his companions to take up residence at the ancient Roman basilica of Santa Sabina, which they did by early 1220.

Hyacinth and Ceslaus along with their companions Herman and Henry were among the first to enter the studium of the Dominican Order at Rome out of which would grow the 16th-century College of Saint Thomas at Santa Maria sopra Minerva and the Pontifical University of Saint Thomas Aquinas, Angelicum in the 20th century.

Establishing a friary at Friesach in Austria, they proceeded to Kraków whence Ceslaus was sent by Hyacinth to Prague, the metropolis of Bohemia.

The city of Wrocław being besieged, the people sought the aid of Ceslaus, who by his prayers miraculously averted the impending calamity.