John Stott, in an excellent interview by Christianity Today, has this to say about Christians and Social Transformation:
Regarding social transformation, I’ve reflected a great deal on the salt and light metaphors, the models that Jesus himself chose in Matthew 5 in the Sermon on the Mount. “You are the salt of the earth; you are the light of the world.” It seems to me that those models must be said to contain at least three things.
First, that Christians are radically different from non-Christians, or if they are not, they ought to be. Jesus sets over against each other two communities. On the one hand there is the world, and on the other hand there is you, who are the dark world’s light. Jesus implied that we are as different as light from darkness and salt from decay.
Second, Christians must permeate non-Christian society. Salt does no good if it stays in the saltshaker. Light does no good if you hide it under a bed or bucket. It has to permeate the darkness. So both metaphors call us not just to be different, but to permeate society.
The third, the more controversial implication, is that the salt and light metaphors indicate that Christians can change non-Christian society. The models must mean that, because both salt and light are effective commodities. They change the environments in which they are placed. Salt hinders bacterial decay. Light dispels darkness. This is not to resurrect the social gospel. We cannot perfect society. But we can improve it.