Liénart had a nickname of a "Red Cardinal" because of his support for the left-wing worker-priest movement and Catholic trade unions, and he strongly promoted social justice within the Church.

[6] Seeking collaboration with workers' associations and pursuing dialogue with socialist and communist trade unions, French worker-priests under Liénart would earn the respect of various left-wing movements, including the Marxist ones.

Liénart continued to maintain the movement even after Vatican took action against worker-priests in 1953, making sure that they could stay active "through special dispensations and broad interpretations of the papal wishes".

[7] Pope Pius XI was committed to 'rechristianizing' the working class in France and downplaying the hitherto elitist image of the Church there; he received a reputation of a "defender of the French proletariat" based on the fact that he appointed Liénart as the Bishop of Lille, baptized Young Christian Workers (JOC), vindicated French Confederation of Christian Workers and published Quadragesimo Anno in 1931 which he focused on the poverty and condition of the working class.

According to Canadian historian Oscar L. Arnal, Liénart established himself as a devoted supporter of both Christian and secular labour unions, and organized several campaigns "to raise money for the strikers’ suffering families, and he had called upon the factory owners to negotiate the labor dispute sincerely".

Instead, they rejected French Revolutionary and liberal values in the name of a more egalitarian society struggling to be born out of a decaying brutal capitalism.

Cardinals like Suhard of Paris, Liénart of Lille, and Gerlier of Lyon believed firmly that worker-priests, proletarian Catholic Action, working-class parishes, and a missionary-trained clergy could offer French workers a serious, socialist alternative to classical Marxism.During the Spanish Civil War, Liénart also organized help and supplies to Basque Country and helped Basque refugese escape to France.

"[9] French right-wing perfume magnate François Coty accused Liénart of "aiding and abetting communism", given his support for trade unions and willingness to negotiate with socialist movements.

Pastoral letters by Liénart were "treated like a social encyclical" in France, and amongst the French left, his actions and willigness to cooperate "deprived their Catholic opponents of any objections in principle".

[11][12] Liénart, assisted by Cardinals Bernardus Johannes Alfrink and Giovanni Colombo, delivered one of the closing messages of the council on 8 December 1965.