There is an extremely interesting article here from a Roman Catholic point of view (it quotes numerous papal encyclicals and appeals to natural and divine law) entitled, "What is Social Justice?" This is an excellent question, and one that has been the bugaboo of my existence since my seminary days.
To make a long story short, I tend to recoil from conversations in the Episcopal church where the term "social justice" comes up as the rationale. At least in the Episcopal realm, the phrase is usually a code words for something else. Namely, it is code for "political agenda," and not just any political agenda, but the political agenda of one particular wing of one particular political party.
Now, let me be clear here. I believe it is perfectly fine to have political opinions. I do firmly believe that sincere, faithful, and informed Christians can legitimately come to different, if not opposing, political beliefs. One is not necessarily stupid simply because another Christian comes to hold a set different political opinions than you. I have no doubt there there were some very sincere Absolute Monarchist Christians back in the day. There probably were, at least initially, some very sincere members of the Nazi Party in Germany during the early 1930's who were also sincere Christians. (Lest we forget that Hitler came to power through completely free and open elections whilst openly campaigning basically on a platform of totalitarianism that promised to wipe away the Weimar Republic and institute a dictatorship, and he still got elected fair and square.)
My point here is that as people of faith, faith in Christ should inform our political beliefs and not vice versa. Our faith should never be our political beliefs in search of religious justification. That, sadly, is exactly how the Episcopal Church currently uses the term "social justice." As someone who is not a liberal Democrat (neither am I a conservative Republican), I was in seminary and still am in my life in the Episcopal church constantly beaten over the head with the term "social justice," which is why I recoil from the term in Episco-speak circles. The term is used as code for a very specific political agenda which my understanding of the Christian faith does not necessarily lead me to accept.
And yet, the argument always goes something like the following:
"You are against (insert culture war issue here for which there is usually no scriptural or sound theological basis)? How can you be against Social Justice?"
Well, I am not against Social Justice per se. I am against your corruption of the term "Social Justice" if your equation is God's Social Justice = Talking Points of Current Platform of the Democratic Party. That's not God's Social Justice, that is your political agenda that you are wrapping up in the guise of the Church.
The same is true for Republicans, except they just use the code term "Family Values," i.e. "How can you be against Family Values, since you don't believe in (insert culture war issue here for which there is usually no scriptural or sound theological basis)?"Again, I am not against Christian family values, depending on how you define them, but therein is the crux.
Using terms like "family values" or "social justice" is manipulative if you are using them to mask your partisan political agenda in non sequitur (that is, "it does not logically follow") argumentation. One of the major themes that Martin Luther King, Jr., who was quite the political theorist in his own right but seldom gets any credit for it, argued in his political philosophy was that "Mankind must look to God." I think Martin Luther King, Jr., in this aspect at least, was correct. We have to look to God to be informed on political matters, and not look to our political matters to be informed about God.
If we are saying what we are doing is "social justice," is what we are advocating really "social" in the Christian sense, and is the justice really from God or something lower. I believe the Lessons of the Weimar Republic stand as a warning to that.