Specifically refers to a bishop in the Catholic Church. Compare 主教, which is used for bishops in other Christian denominations (Anglican, Orthodox, etc.).
See also: 主教
The Pope appointed a new bishop.
The Pope appointed him as the new bishop.
After many years of service, he became a bishop.
主教 refers to a bishop in Orthodox, Anglican, or other non-Catholic churches, while 司教 specifically denotes a Roman Catholic bishop.
From 司 (director, administer) + 教 (teaching, religion), a term coined for the Catholic hierarchy.