Complex sentence: He served as a priest until 1942 when he then worked for the Vatican Secretariat of State until 1959 when Pope John XXIII appointed him Bishop of Angers.
Split into simpler sentences: He served as a member of the parochial clergy until 1942, when he went to work in the Vatican Secretariat of State. In 1959 Pope John XXIII appointed him Bishop of Angers.