[30] He concentrated initially on diocesan organization, establishing the independence of the Church from government authorities who had heretofore been allowed to interfere, and creating societies for the men and women in each parish.

[32] He made his ad limina visit to Rome en route to Paris and was honored with the title Assistant at the Papal Throne on 28 March 1854.

[33] He spent most of his time in France, but also visited Rome to participate in the proclamation of the doctrine of the Immaculate Conception on 8 December 1854 and the consecration of the Basilica of Saint Paul Outside the Walls two days later.

After returning to Réunion, on 9 October 1856 he laid the cornerstone of the diocesan cathedral, a festive event in which the new governor of the colony participated to demonstrate that he did not share his predecessor's antagonistic attitude toward the Church.

[28][36] In 1861, he completed the plans of his predecessor to use the Roman liturgical rite in Toulouse,[6] part of the long process of suppressing Gallicanism and promoting uniformity of practice.

[38] In 1862, Desprez provoked a firestorm of protests by proposing a religious observance to mark the 300th anniversary of events that occurred in Toulouse during the Wars of Religion, when Catholic forces massacred several thousand Huguenots who had been granted safe conduct.

[39] The Journal des débats reviewed the events of 1562 in detail and called Desprez's planned festival "a deplorable anachronism and a danger to the public peace".

[41] After the French government forbade the publication of Pope Pius IX's Syllabus of Errors condemning modernism, Desprez protested the Church's right to publish it.

[42] He pressed the case for the canonization of the recently beatified Saint Germaine of Pibrac, a 17th-century shepherd girl abused by her step-mother in a village just west of Toulouse.

In addition to theological studies, Desprez emphasized its mission in opposition to secularism and contemporary society: "What is the purpose of founding Catholic universities?

"[44][45] Pope Leo XIII made him a cardinal priest on 12 May 1879[9] and gave him his red galero and the title of Santi Marcellino e Pietro on 22 September 1879.

[47] A doctor who worked in Toulouse during the cholera epidemic of 1885 reported that Desprez was "the only official personage who had the courage at that time to cross the perilous threshold" of the lazaretto.

He also reported being told several times that Desprez was the biological son of Emperor Napoleon, though "it was pretty generally agreed that he had not inherited the great intellectual gifts of his supposed father".