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Why was The Spirit of Laws placed on the Index of Forbidden Books in 1751?

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Oval portrait print of Montesquieu by Pierre Michel Alix after Jean Francois Garnerey, showing Montesquieu with long curly white hair and a fur-trimmed robe, facing forward. — Montesquieu Montesquieu.

The Spirit of Laws was placed on the Index Librorum Prohibitorum (Index of Forbidden Books) in 1751 due to the efforts of Montesquieu’s enemies within the Roman Catholic Church, who viewed the work as dangerous to the faith or morals of Catholics.12

While Montesquieu faced and successfully deflected initial attacks from the Sorbonne and the general assembly of the French clergy, his opponents in Rome ultimately succeeded in having the book proscribed. This occurred despite the intervention of the French ambassador, the support of several liberal-minded high ecclesiastics, and the personal favorable disposition of Pope Benedict XIV.13

The Index was a tool used by ecclesiastical authorities to prevent the spread of works deemed theologically erroneous or harmful to the faith. Although the inclusion of his masterpiece on the list was a source of dismay for Montesquieu, it proved to be only a momentary setback, as he had already published his Defense of the Spirit of Laws in 1750 to address such criticisms.213