At the age of thirteen he became a convert to Roman Catholicism, and when he left the Lycée Henri IV in 1822, he became a disciple of Auguste Comte, who initiated him into the doctrines of Saint-Simon (and later Barthélemy Prosper Enfantin), to the propagation of which he devoted a part of his fortune.
In 1832 d'Eichthal went to Greece, and on his return to Paris in 1836 published "Les Deux Mondes," containing his reflections on the Orient. He now began to advocate the use of Greek as a universal language, and published many works, among which were: "Les Trois Grands Peuples Méditerranéens et le Christianisme," Paris, 1864; "Origines Bouddhiques de la Civilisation Américaine," in the "Revue Archéologique," Sept., 1864, and April, 1865; and "Texte Primitif du Premier Récit de la Création," Paris, 1875; reprinted after his death under the title "Mélanges de Critique Biblique."
He had two sons, Eugène Séligmann d'Eichthal, born November 3, 1844 in Paris, died February 28, 1936 in Paris; and Georges Séligmann d'Eichthal, as well as a daughter whose married name was Mme. Paul Le Bret. She had been a friend of Rosa Bonheur when they were both children, as their fathers had both lived briefly at the Saint-Simonian monastery at Ménilmontant.