In 1597, he was appointed Protonotary apostolic, member of the college of protonotarii apostolici de numero participantium which was facilitated by his kinship with Cardinals Marcantonio and Ascanio Colonna.

[2] In 1630 in Paris, Cardinal Guidi di Bagno met Gabriel Naudé who became his librarian and secretary and accompanied him in 1631 to Italy,[6] and René Descartes highly appreciated him.

[2] Many contemporaries were convinced that [Guidi di Bagno] had all the qualities to be a worthy successor to Urban VIII.

When Urban VIII fell seriously ill in 1637, Richelieu had [Guidi di Bagno] proposed as the official candidate for France.

He was undoubtedly appreciated and regarded in many circles; he had the ability to behave independently in conflict situations and to maintain friendly relations even with personalities of different political persuasions.