[clarification needed][2] An ardent republican, he strongly supported Collot d'Herbois' motion for the abolition of the monarchy in the first session of the National Convention (21 September 1792) with the memorable phrase "Kings are in morality what monsters are in the world of nature.

During the trial, being absent with other three colleagues on a mission for the union of Savoy to France, he along with them wrote a letter urging the condemnation of the king, but attempted to save the life of the monarch by proposing that the death penalty should be suspended.

[4] In a series of three reports issued to the National Convention in 1794, Grégoire advocated for additional protection of art works, architecture, inscriptions, books, and manuscripts.

The Abbé Grégoire is also known for advocating a unified French national language, and for writing the Rapport sur la Nécessité et les Moyens d'anéantir les Patois et d'universaliser l'Usage de la Langue française (Report on the necessity and means to annihilate the patois and to universalise the use of the French language),[5] which he presented on 4 June 1794 to the National Convention.

[7] However, his work was still influenced by the rising sense of French linguistic superiority that had been started by Bertrand Barère with Rapport du Comité de salut public sur les idiomes (1794).

In October 1789, Grégoire took a great interest in abolitionism after he had met Julien Raimond, a free colored planter from Saint-Domingue who was trying to win admission to the Constituent Assembly as the representative of his group.

It was on Grégoire's motion in May 1791 that the Constituent Assembly passed its first law admitting some wealthy free men of color in the French colonies to the same rights as whites.

He blamed the condition of the Jews on the way they had been treated, their persecution by Christians, and the "ridiculous" teachings of their rabbis, and believed they could be brought into mainstream society and made citizens.

He was a leading voice at the national church councils of 1797 and 1801; but he was strenuously opposed to Napoleon's policy of reconciliation with the Holy See, and after the signature of the Concordat he resigned his episcopal see on 8 October 1801.

This was considered a potentially harmful episode by the powers of the Quintuple Alliance, and the question was raised of a fresh armed intervention in France under the terms of the Treaty of Aix-la-Chapelle.

To prevent this, Louis XVIII decided on a modification of the franchise; the Dessolles ministry resigned; and the first act of Count Decazes, the new premier, was to annul the election of Grégoire.

Hyacinthe-Louis De Quelen, the uncompromising royalist Archbishop of Paris, would only concede on condition that he retract his oath to the Civil Constitution of the Clergy, which Grégoire refused to do.

Its clergy absented in obedience to the archbishop's orders, and Mass was sung by the Abbé Louis-Charles de Grieu (1755–1836), assisted by two clerics, the catafalque being decorated with the episcopal insignia.