Christian socialism generally refers to those on the Christian left whose politics are both Christian and socialist and who see these two philosophies as being interrelated. This category can include Liberation theology and the doctrine of the social gospel. The term "Christian Socialism" is used in this sense by organizations such as the Christian Socialist Movement (CSM). The term also pertains to such earlier figures as the nineteenth century writers Frederick Denison Maurice (The Kingdom of Christ, 1838), Charles Kingsley (Water-Babies, 1863), Thomas Hughes (Tom Brown's Schooldays, 1857), Frederick James Furnivall (co-creator of the Oxford English Dictionary), Adin Ballou (Practical Christian Socialism, 1854), and Francis Bellamy (a Baptist minister and the author of the United States' Pledge of Allegiance).
On the Catholic side, the Rerum Novarum encyclical letter of Leo XIII (1891) was the starting point of a Teaching on social questions that was expanded and updated all through the 20th century. Though avoiding the word Socialism (as the Socialist movements of the day were anti-religious) the encyclical promotes a kind of corporatism based on social solidarity among the classes with respects for the needs and rights of all. In the more Catholic countries of Europe the encyclical's teaching was the inspiration that led to the formation of new Christian-inspired Socialist parties.
A number of Christian socialist movements and political parties throughout the world group themselves into the International League of Religious Socialists. It has member organizations in 21 countries representing 200,000 members.
Christian socialists draw parallels between what some have characterized as the egalitarian and anti-establishment message of Jesus, who according to Christian Gospel spoke against the religious authorities of his time, and the egalitarian, anti-establishment, and sometimes anti-clerical message of most contemporary socialisms. Some Christian Socialists have gone as far as to become active Communists (see Christian communism). This phenomenon was most common among Christian missionaries in China, the most notable being James Gareth Endicott, who became supportive of the struggle of the Communist Party of China in the 1930s and 1940s.
Christian socialism is not to be confused with certain parties with "Christian Social" in their names which are found in the German-speaking world, such as the contemporary Christian Social Union in Bavaria or the Christian Social Party in Austria-Hungary circa 1900. Such parties do not claim to be socialist, nor are they considered socialist by others. The term Christian Democrat is more appropriately applied to the contemporary parties.
The British Labour Party and Australian Labor Party have both been influenced by Christian socialism, and some figures from both parties could be considered to be Christian socialists, depending on the definition of 'socialism' used.
Former British Labour leader Tony Blair is a member of the Christian Socialist Movement although his adherence to Christian Socialist ideals are highly disputed, as he is much farther to the right than most "socialists."
Australian Prime Minister Kevin Rudd identified himself as an "old-fashioned Christian socialist" in a 2003 interview with the Australian Financial Review , later writing in 2006 that "A Christian perspective, informed by a social gospel or Christian socialist tradition, should not be rejected contemptuously by secular politicians as if these views are an unwelcome intrusion into the political sphere." However he also described socialism as an "arcane, 19th century" doctrine and stated that "I am not a socialist. I have never been a socialist and I never will be a socialist."