Tim Keller is the go to guy on this. He really launches from the Prophets and develops an optimistic theology of the city. I find him very convincing. Others who argue this would be Andy Crouch in his book Culture Making. Sadly the majority who do this are left leaning... or at the least accused of being liberal and positing a social gospel. Herman Ridderbos in The Coming of the Kingdom strongly denied such an idea, saying the a gospel for the welfare of all people is thoroughly consistent with reformed theology.
The reformed gospel of Jesus Christ celebrates His exaltation, but often at the expense of the incarnation. Having a robust Christology propels us to engage the culture seeking to glorify God. The two men I mentioned, along with Mark Driscoll, argue that the church's primary responsibility is the preaching of the word and the ministry of sacraments. But let us not forget the missionary calling of the church as well. John Piper wrote, "missions exist because worship does not." The two are intrinsically connected.
When Christians fight for social justice we are not abandoning the gospel of Christ... for it is a gospel of the coming kingdom. Social justice by definition for the Christian, this is off the top of my head, is to seek the welfare of all people. Look at Daniel and Esther for instance. Daniel seeks the welfare and blessing of the pagan ruler Nebuchadnezzer. The word commends him for that. Social justice as an end to itself is folly, look at the conclusion of Esther where taxes rise, for this is a sinful world. But we as Christians are to seek the welfare of all people, for the glory of God. Shallom will only come with the gospel of Jesus Christ. Shallom can only be demonstrated within the open, loving family relationship of the church.
But what does this look like? Satz is right.. the OT does not spell that out for us, instead it is left to our discernment. Abortion rings in my mind. As does euthanasia, infanticide, sex trafficking, the OT condemns poverty as it is an earmark of sin and the fall, inequality, and classism. These are only a portion of the social injustices to fight.
Look at Tim Keller, Francis Schaeffer, Herman Ridderboss, Mark Driscoll, John Piper, Mike Horton and Tim Chester, who has a blog and wrote several books, for resources.
On the note of Horton the doctrine of the 2 kingdoms is helpful. It does not matter, before the throne of God, if one is politically left, right, or centrist. Christians are to fight social injustice, not because that is the party line, but because if glorifies God. And in some way when we do this a glimmer of heaven is seen through "the glass darkly."
