It comes from a need to support slaves, or weak powerless people.
Slaves as a term isn't as relevant today as it was centuries ago, but still it captures the idea of a class of "oppressed and powerless" people well.
In an extreme twist of irony however, both Christianity and Judaism are religions born from slaves, hence the emphasis on classic liberal values (love thy neighbor as thyself). Islam however is not a slave religion, and was born from power, on the other end of the spectrum (kill non-believers who don't convert)
Slaves as a term isn't as relevant today as it was centuries ago, but still it captures the idea of a class of "oppressed and powerless" people well.
In an extreme twist of irony however, both Christianity and Judaism are religions born from slaves, hence the emphasis on classic liberal values (love thy neighbor as thyself). Islam however is not a slave religion, and was born from power, on the other end of the spectrum (kill non-believers who don't convert)