David Powlison uses a helpful allegory to describe the purpose of social justice in the grander scope of the gospel:
Imagine the armies of darkness as well-armed, dedicated, brainwashed soldiers who are barefoot, ragged, diseased, hungry, wounded and oppressed by their officers. Against them the task of the armies of light is to fight in two modes, first destroying their moral evil and second doing them good to overcome situational suffering. The armies of light drop bombs and leaflets on the soldiers [. . .] to destroy moral evil. [. . .] The army of light also makes airdrops of food, clothing, shoes, and medicine behind enemy lines. Life-giving supplies [. . .] make the promises [of God] tangible by actually doing some immediate good.
The curse caused by humanity’s sin not only destroys us all morally, but also causes all suffering, pain, hurt, hunger, disease, and oppression. While Christ died to put an end to suffering, his primary concern is to save us from our moral evil.
When we work for social justice we incarnate the gospel by relieving suffering. We restore people’s bodies from disease, their families from poverty, and their stomachs from hunger to illustrate how Christ’s gospel redeems our moral problem.
Jesus put it this way,
‘Which is easier, to say to the paralytic, ‘Your sins are forgiven,’ or to say, ‘Rise, take up your bed and walk’? But that you may know that the Son of Man has authority on earth to forgive sins’—he said to the paralytic— ‘I say to you, rise, pick up your bed, and go home.’(Mark 2:9-11 ESV)
Social justice is never an end, but a means by which the Holy Spirit whets our appetite for salvation from our sins. The salvation revealed in the gospel is the most true salvation, saving us respite from eternal suffering. By this salvation we gain eternal life, which is to know God, share in his glory and revel in joy far greater than what our imaginations can fathom.