Social Justice is a value mandated by the Gospel.
In my essay "Atonement: Solidarity and The Basis of Social Justice," I demonstrated that the most efficacious model for understanding atonement of Christ on the cross for our sins is that of a statement of solidarity between God and humanity,When we accept that statement of God's solidarity with us we are expressing our solidarity with God thus putting ourselves in a relationship of solidarity, aka "covenant," that creates the grounds upon which sin is forgiven. The title implies that there is a further implication of social justice, but I did not spell it out, I will do so here: there is a social dimension of the gospel mandates social justice as priority value and legitimizes the Christian mission for work for social justice.
This is related to the solidarity aspect, which is another way of speaking about covenant. Paul clearly implies that there is a social dimension of the Gospel. In the book of Galatians Paul lays out the way of Grace and it;s implications in relation to law. He proclaims the Gospel of grace as the true Gospel and pronounces a curse on anyone who teaches another gospel, (Gal 1:8), In chapter 2:14-15 Paul confronts Peter for what he sees as hypocrisy. His withdrawing from the gentiles when the representatives of James came, out of fear of their anti-gentile prejudice.
14 When I saw that they were not acting in line with the truth of the gospel, I said to Cephas in front of them all, “You are a Jew, yet you live like a Gentile and not like a Jew. How is it, then, that you force Gentiles to follow Jewish customs?
15 “We who are Jews by birth and not sinful Gentiles 16 know that a person is not justified by the works of the law, but by faith in Jesus Christ. So we, too, have put our faith in Christ Jesus that we may be justified by faith in Christ and not by the works of the law, because by the works of the law no one will be justified.
After a long section in chapter two where he justifies the concept of salvation by grace through faith, and sets that forth as the essence of the Gospel, this is what it's all about, he goes on in chapter 3 to lay out the implications and he puts on it a class a analysis: (3;26) "so in Christ you are all children of God Through Faith, 27 for all of you who were baptized into Christ have clothed yourselves with Christ. 28 There is neither Jew nor Gentile, Neither sla e nor free, nor is there male or female, for you are all one in Christ Jesus. 29 If you belong to Christ, then you are Abraham's seed, heirs according to the promise.."
I have seen conservative Christians relegate this concept to the back burner several times, none more infuriating than a time when I lived in New Mexico (long time ago) . At a house discussion group connected with my church we had been discussing "male/female roles in the church," They agreed all comers welcome in discussion, They were going to listen to my presentation of my my view blah lblah all fair sure right on. I laid it all out it meant nothing to them. I kept harping on Galatians 3:26-29 the main leader of the "conservatives" says "that;s just before the Lord." What he meant was this sounds good Chrisianitistically but in "real life:" where people live it is means nothing. This guy was a great man of faith. But he could not deal with they fact that in the concrete world where Paul lived Peter was acting hypocritically to perpetuate the social distinction grounded in the law between Jews and gentiles. It is a real world concrete implication of the Gospel. Paul lays it bare; it's a matter of "the truth of the Gospel: and of "belonging to Christ." Gal 2:14 and 3:29.This does mean if you are trying to justify racism by the gospel you are preaching another gospel you do not belong to Christ and you are under a curse. It probably also mean we can't make distinctions about rank in the church based upon gender, There are no second class christians and if you believe in Jesus there is a social application to the gospel.
Since this is grounded in the basic principles of the Gospel dealing with grace and freedom (Gal. 5:1 states it as a positive primary value "It is for freedom you have been set free") then it must be grounded in the concept of solidarity, It only makes sense, how could one be in solidarity with God, who is in solidarity with humanity and not be in solidarity with humanity as well? Jesus shows a preference for the poor in identifying with the bottom rung of society in the way he lived and the way he died, crucified between two thieves, Not only was his death reserved for criminals but also for rebels against Rome, He was traded for the revolutionary Barabbas who was freed in his place, That has obvious literary meaning. Jesus was the more dangerous to the powers of the day, The Elder John put's it in his own way, how can you love God who you have not seen (1 john 4:20)
Solidarity is about identifying with the other. Jesus identified with humanity and with the bottom of the social ladder. In the most basic statements of the Gospel in the early days of it's proclamation reflect this aspect; Luke is best about reflecting that aspect, In Luke 4:16-21 Jesus stands to read from Isaiah in the synagogue he is essentially introducing himself (even though it;s his home town) and unavailing his true mission,He quotes from Isaiah (Isaiah 61:1,2 (see Septuagint); Isaiah 58:6):
“The Spirit of the Lord is on me,
because he has anointed me
to proclaim good news to the poor.
He has sent me to proclaim freedom for the prisoners
and recovery of sight for the blind,
to set the oppressed free, to proclaim
the year of the Lord's favor
James Tabor Links these two aspects the resurrection of the dead and preaching of good news to the poor thus marking both as central features of identification for the messiah: Jesus might as well have said:hey I'm the messiah,"
when we turn to the Q Source, which Luke and Matthew quote, regarding the "signs of the Messiah," we find the two phrases linked: "the dead are raised up, the poor have the glad tidings preached to them," precisely as we have in our Qumran text. Luke makes more than passing use of this notion of the "resurrection of the dead" as a sign of the age of the Messiah. In the two places he quotes Isaiah 61:1 he also mentions specific cases of resurrection of the dead: as Elijah once raised the son of the widow, Jesus now raises the son of the widow from Nain (Luke 4:26; 7:11-17). This is hardly accidental, as the close juxtaposition of the texts makes clear."
He is basing this upon the use of the Isaiah phrases in the Qumran literature, for details more of the context and analysis see my article on Doxa  Walter Pilgrim Understood a social implication to these passages. An SIM* task force om Social justice makes a finding on this passage:
No doubt, this text is one of the clearest statements of Jesus’ mission and the goals of his ministry. It is also one of the most misunderstood. In popular explanations, Luke 4 underscores that Jesus’ mission focused on the materially destitute and the downtrodden. In this interpretation, Jesus is Messiah and social liberator. He came to bring the year of jubilee to the oppressed. He came to transform social structures and bring God’s creation back to shalom. Therefore, our mission, in keeping with Christ’s mission, is, to quote one well-respected book “to extend the kingdom by infiltrating all segments of society, with preference given to the poor, and allowing no dichotomy between evangelism and social transformation (Luke 4:18-19).” Above all else, Luke 4, it is argued, shows that Jesus’ mission was to serve the poor. Shouldn’t that be our mission too?*
That is the popular misunderstanding, What is misunderstood? The idea that"poor" is limited to financial poor. The finding of the Task force is that the Greek word ptochos
Social injustice is a social dynamic stemming from sin, Social injustice is a from of sin a is any other transgression such as adultery or stealing. The social dynamic makes it easier to rationalize. Reinhold Niebuhr apples class analysis and finds that the class dimension to the social dynamic creates personal identification with class interest and thus an easy rationalization as the interest of the larger group becomes confused with one's personal interest, This is the meaning of his title, Moral Man and Immoral Society.  The individual is capable of being fairly moral although one always struggles with temptation, Yet with the social and class dimensions the rationalization factor sucks one into teh mob mentality It then becomes the Christian's duty to soul search and part for conviction to realize the out of the morass. Actually that part is my interpretation. Niebuhr just sort of assumed that being aware was enough.
We need to go one step further, however, and not content ourselves with merely being aware of our identification and rationalization of social sin. That's the leftist complaint about the liberal; it's all about our personal feeling good. We need to work for social justice. After all what good does it do to just be aware if we are aren't really working to change things we are not really sorry about our contribution to the problem. The major brunt of social sin comes down on the poor who are the primary victims. That is exacerbated times whatever other marginalization factors obtain, In other words "the poor" are having it bad. But a poor person who is being oppressed for being gay is having it even worse. Poverty is connected both as cause and implication to all forms of social ill Poverty is decisive in many way for life chances.  
Poverty is the symbol for all social sin and deserves our highest effort to fight it, Politicians who flagrantly disregard on a par with murderers. Those who rationalize the system are not acting in line with the truth of the Gospel. Fighting for social justice is as much our Christian duty as spreading the gospel,
 Joseph Hnman, "Atonement, Solidaroty, and Social Justice, part 1" Metacrock's blog
MARCH 05, 2017, URL:http://metacrock.blogspot.com/2017/03/the-atonement-solidarity-and-basis-of.html
 James D. Tabor, Archaeology and The Dead Sea Scrolls"The Signs of the Messiah: 4Q521"
The Jewish Roman World of Jesus (website) (accessed 3/12/17)
 Joseph Hinman, "refutting the 'No body' Theroy," Resurrection pages,Doxa:Christian Thouht in the 21st Centiry website URL:
http://www.doxa.ws/Jesus_pages/Resurrection/Res_body.html (accessed 3/12/17)
Walter Pilgrim, Good News to the Poor: Wealth and Poverty in Luke-Acts, Eugene Oregan:Wipf and Stock publishers, 1981, 15, 65-66.
prof of theology pacific Lutheran University
 Kevin De Young, "Seven Passages on Social Justice," The Gospel Coalition, (July 20,2010)
on line resource, URL:
is senior pastor of University Reformed Church (PCA) in East Lansing,
Gordon-Conwell Theological Seminary.
from the parent document:
NOTE: This is an abbreviated version of a much larger work originally undertaken by a task force appointed by the Board of Governors of SIM* to investigate issues of social injustice in the context of Christian mission. The paper produced is not a final statement on justice or an SIM position paper, and does not reflect the beliefs and opinions held by all SIM members. The convictions presented represent the interplay between the various authors’ biblical understanding, cultural worldview, and personal perspectives. This has been a very helpful process for SIM to work through. Malcolm McGregor, SIM International Director following URL link to PDF of Task force report:
*SIM = Soudan Interior Mission, a mission society began in 1893. It's still going. It produces sophisticated scholarly literature, it has 4000 workers in 70 country, it's members represent as many nationalities.
 Reinhold Neibuhr, Moral Man and Immoral Society, Westminster John Knox Press; 2nd ed. edition (January 18, 2013)
 Jeanne Brooks-Gunn and Greg J. Duncan, "The Effects of Poverty on Children," The Future of Children (Summer Fall 1997), 55-71. on line version URL:https://www.princeton.edu/futureofchildren/publications/docs/07_02_03.pdf (accessed3/12/17)
Jeanne Brooks-Gunn, Ph.D., is Virginia and Leonard Marx professor of child development and education, and is director of the Center for Young Children and Families at Teachers College, Columbia University. Greg J. Duncan, Ph.D., is a professor of education and social policy, and is a faculty associate at the Institute for Policy Research, Northwestern University
 GILLIAN B. WHITE,"Poverty Compounded," The Atlanic (APR 16, 2016)