Complex sentence: In 1924 he resigned from his chair to concentrate on his political career, and, in 1925 to take up the post of Secretary to Bishop Nuncio Eugenio Pacelli-later Pope Pius XII.
Split into simpler sentences: In 1924 he resigned from his chair to concentrate on his political career. And, in 1925, he took up the post of Secretary to Bishop Nuncio Eugenio Pacelli-later Pope Pius XII.