The following excellent explanation of liberalism is taken from: Parente, Pietro; Piolanti, Antonio; and Garofalo, Salvatore, Dictionary of Dogmatic Theology, translated by Emmanuel Doronzo, O.M.I., S.T.D., Ph.D. (Milwaukee: The Bruce Publishing Company, 1951) p. 163 and 164. I have provided only the linked references and one small note in brackets.
Liberalism. A doctrinal current, quite complex and changeable, which has had various interpretations and practical applications, not easily definable. The basic concept of liberalism is liberty, taken as emancipation and independence of man, society, and State, from God and His Church.
Born of Encyclopedism, liberalism finds a philosophical justification in Kantianism (q.v.), and gains strength with naturalism and rationalism (q.q.v.); with the French Revolution it enters the sociopolitical field and manifests itself as exaggerated democracy (sovereign people), as separatism with respect to the relations between Church and State (“A free church in a free State”), as indifferentism in matters of religion and worship, and as abstentionism (noninterference) of the State in economics (“Leave everything to private initiative”) [BAM note: this form of liberalism is known as "libertarianism" in American politics].
In the first half of the past [19th] century this dangerous and erroneous current made great inroads among Catholic ranks, assuming a more moderate form and insisting especially on the separation of Church from State and on broad-mindedness with regard to a liberal spirit. Characteristic in this connection was the Catholic-Liberal movement in France, led by Félicité de Lamennais, and followed enthusiastically by Lacordaire, a Dominican, Montalembert, and others. These sought, with the best of intentions but to no avail, to Christianize liberalism, fundamentally adverse to revealed religion. The Church was forced to intervene, first warning, then condemning.
The principal documents of the Church magisterium are: (I) The encyclical, Mirari vos, of Gregory XVI (1832). (2) The encyclical, Quanta cura, with the attached Syllabus, of Pius IX (1864). (3) The encyclicals, Immortale Dei and Libertas, of Leo XIII (1885 and 1888).
In the Syllabus (q.v.) is found the explicit and detailed condemnation of liberalism, whether philosophical, theological, religious, or sociopolitical. Certain modern tendencies with a more attenuated liberal tinge are to be distinguished from this classic liberalism.
Leo XIII, in his two famous encyclicals, confirms the condemnation given by Pius IX in the Syllabus, maintaining vigorously the rights of God and of the Church with regard to the individual and the State, which cannot divest itself of interest in the religious problem or put the Catholic Church on a par with other cults. But, in consideration of contingent difficulties, he does not condemn the government which, for reasons of freedom of conscience, permits in its territory — even where the majority of citizens is Catholic — the free exercise of other religious forms. This is a tolerance, therefore, of practical necessity, similar to that with which God tolerates evil by the side of good in the world; but the principle remains intact, namely: the truth and the right of the Catholic religion and Church in its relations with the individual, with society, and with the State.
BILLOT, De Ecclesia Christi, Vol. 2 (Rome, 1922), pp. 15—58. CONSTANTIN, “Libéralisme,” DTC. DE PASCAL, “Libéralisme,” DA. GIBBONS, Faith of Our Fathers (Baltimore, 1890) GRUBER, “Liberalism,” CE. MANNING, “Liberty of the Press,” Essays, third series (London 1892). MING, Data of Modern Ethics Examined (New York, 1897). WEILL, Histoire du catholicisme libéra1 en France 1828—1908 (Paris, 1909).