Quick answer
Callistus is a boy name meaning "most beautiful".
Pronunciation: English /kə.ˈlɪs.təs/.
Meaning and history
Late Latin name that was derived from the Greek name Κάλλιστος (Kallistos) meaning "most beautiful". This was the name of three popes (also known as Callixtus), including the 3rd-century Callistus I who is regarded as a saint.
Notes and context
Usage: late roman