In this new post, he won the commitment and confidence of those with whom he disagreed and protected démissionnaires from over-exultancy, becoming known in Castelnaudary as le Bon curé.

M. Guillaume Besancel, constitutional bishop of the Aude, rendered unable to carry out his duty by infirmities and old age, stated his desire to have a coadjutor.

It was known that many of these prelates (including Belmas) were refusing to sign a retraction that the legate would demand of them – they declared only that they renounced the civil constitution of the clergy, condemned by the Holy See.

When France's break from Rome was finally ended by the combined efforts of pope Pius VII and first consul Napoleon, Belmas was appointed to the seat of Cambrai (now only a diocese under the metropolitan of Paris rather than an archdiocese of its own) on 11 April 1802.

Such work was vast and near-impossible, with no unity or discipline among its clergy, not a single priestly establishment and no monetary resources, but (far from being discouraged) Belmas's zeal and energy was only redoubled by such obstacles.

The tomb containing the body of Fénelon was rediscovered in 1804 and Cambrai's magistrates resolved to transfer the remains into the chapel of the hospice de Sainte-Agnès.

Following Napoleon's deportation of the pope to Savona, a church council was convoked in Paris in 1811 to remedy some of the problems Catholicism was having in France – Belmas attended it but seems not to have taken an active part.

It was proposed that he resign, but he would only consent to that if they give him the title of archbishop in partibus, which the pope was unwilling to do, and so Cambrai's promotion to an archdiocese was adjourned by a papal bull of October 1822.

Until his last day Belmas worked hard and with presence of mind for his diocese, still having his correspondence read to him and dictating his replies until shortly before his death.

The see was promoted to an archdiocese after Belmas's death by a bull of pope Gregory XVI on 1 October 1841 in favour of Pierre Giraud.

Casimir-Alexis-Joseph Wicart (then curé-doyen of Sainte-Catherine in Lille, later bishop of Fréjus) led Belmas's funeral in Cambrai, with the whole town in mourning.