The social gospel is an idea that arose particularly in Protestant denomination churches in the US and in parts of Europe during the 19th century. It was the concept that efforts should be made to share wealth, and especially that people should emulate Christ’s examples by acting with charity toward those less fortunate. Much of the modern progressive stance is tied to social gospel concepts, though these may have been divorced from the religious aspects originally associated with the movement.
People who initially came up with ideas that became part of the social gospel, like Richard Ely, were deliberately countering one of the most popular theories of human development of the time: Social Darwinism. This unapproved extrapolation of Darwin’s theories that the fittest members of society would survive and prosper gave license to many people to argue they could commit all manner of abuses toward others who were habitually the weakest. Growing industrialization led to development of cities and those interested in the social gospel were so sickened by poverty and horrendous living conditions of humans in urbanized environments that they felt it was their responsibility as Christians to intervene and help improve life for others, through methods like direct charity, education, and sometimes conversion.
Though the social gospel is often compare to socialism, it really was not about surrendering all property and sharing with others. People like Ely advocated for private ownership but also staunchly insisted that a specific duty of humans was treating the less fortunate with care, perhaps by giving up a little, though not all, of personal comforts. The idea of “What would Jesus do?” arises from this viewpoint.
In the 20th century, the social gospel fervor died back principally by the end of WWI, but it had some noted flares of excitement as the century wore on influencing some aspects of the New Deal and then the ideas of Martin Luther King, Jr. Those who marched in support of Civil Rights were principally reaching with the same intent and fervor toward equality for all as those people who had espoused the original gospel ideas nearly 100 years before. Part of the Democratic Party continues to reflect ideas of the social gospel, especially in supporting things like public education, welfare, food stamps, and other social works.
While there are many individual Christian churches that may still be guided by the social gospel, the idea is now often more secular than sacred, and it is divorced from Christianity sometimes completely, though politicians especially may argue they support progressivism because of deep religious convictions.