Andy Crouch has the best discussion of the relationship between evangelism and social action that I have ever read. You can find it in an excursion at the end of chapter four in his new book Playing God: Redeeming the Gift of Power (IVP, 2013). I have copied two of my favorite paragraphs below.
As for the question of why we should work for justice in a world that is passing away—well, the world is passing away. Our work for justice should no more be based on the idea that humanity will somehow progress on its own merits to utopia than proclaiming the good news about eternal life should be based on the idea that someone who accepts Jesus into their heat will never die. The Christian hope is not for a gradually improving world any more than it is for a fountain of youth. But Christian hope overcomes the forces of despair and decay in the midst of this world, and provides foretastes of the coming kingdom where anyone who will receive the Lamb’s sacrifice will be raised to life, and where the glory and honor of the nations will be presented as offerings to the King of kings. Hope for a life beyond this life, and a world of shalom beyond this world of injustice, is the greatest resource for the work of justice here and now. Christian hope for a world made new is not an alternative to doing justice—it is the most essential resource for it.
Christians who truly want to seek justice cannot afford to let “justice” be reduced to the lowest common denominator we may be able to agree on with our neighbors. To do that would be to surrender to gods that are not real gods—to assent to the serpent’s promise that apart from relationship with God we can be like God, knowing good and evil. We can work for common goals for uncommon reasons. Because we believe every one of our neighbors is an image bearer, however broken their relationship with the One whose image they bear, we will find much common ground for working for justice and freedom. The things our neighbors seek are good; they are just not ultimate goods. We can work alongside them for the good while worshiping the One who alone is good.
I would highly recommend you pick up a copy of Playing God. For a fine classic treatment on this topic, check out John Stott’s 1975 book Christian Mission in the Modern World, which was reprinted with a new introduction by Ajith Fernando in 2008. The latter was written in the wake of the Lausanne Congress on World Evangelization in 1974.