Social Justice is a major part of the Christian tradition. It forms one of the central pillars of moral theology. Contrary to the way some speak of social justice, it is not a new feature, but rather, it comes from the moral teachings found in the Torah, the Prophets, the Wisdom and Historical Books, and from the preaching of Jesus and his first Apostles. It continued to be found in patristic writers, with some like Salvian taking particular interest in it and making it central to their writings, with others like St. Basil, St. John Chrysostom, St. Cyril of Alexandria, et. al., following through with it as needed, usually in and through their commentaries and homilies on Scripture, but also in relation to their civic and religious duty when society had to be brought together to work for the common good (such as in times of extreme famine).
Social justice must not be misconstrued as “misguided compassion.” While compassion certainly should be a part of the equation, because justice is not justice without mercy and grace, without transcendent charity penetrating it and making it more than another form of legalism, justice is about righteousness and making less-than just situations just. Compassion is a tool which helps make people realize where injustice is occurring, and sympathize with the victims of injustice, but even without compassion, even without charity, a mere desire for righteousness itself should suffice as to justify social justice and demonstrate why it is a necessary part of Christian moral teaching. Social justice is not some sort of liberal relativism deny objective goodness, rather, it follows objective goodness as a reason why justice must be followed, showing that it is one of the most conservative elements of the Christian tradition.St. Jerome, in his commentary on Isaiah, rightfully explained that justice and morality are intertwined:
To what is said here, Keep judgment, and do justice, the following words are similar: “Blessed are those who keep judgment and do justice at all times” [Ps 106:3], so that they justly pursue what is just – although under the name of justice all points of morality appear to be to be signified. For the one who does a single justice is shown to have fulfilled all the virtues, which follows each other in succession and cleave to each other. Consequently one who has one, has them all, and the one who lacks one, lacks them all. 
St. Jerome’s point follows what James wrote, when he said, “For whoever keeps the whole law but fails in one point has become guilty of all of it” (James 2:10 RSV). Social Justice realizes the unity of all moral claims, of all justice, that all that is good is united together as one holistic doctrine, one holistic good, and all sin is about the destruction of the “seamless garment” of truth which Christ used to cover the church in glory. Social justice realizes this is true, not just for people as individuals, but for people in their personal relationships, in their communities, in the structures which they implement in society. If those structures are unjust, then they are sinful and must be overturned.