John Stott has one of the best answers to this, in my view. From his article Four Ways Christians Can Influence the World:
If we are pessimists and think we are capable of doing nothing in human society today, I venture to say that we are theologically extremely unbalanced, if not actually heretical and harmful. It’s ludicrous to say Christians can have no influence in society. It’s biblically and historically mistaken. Christianity has had an enormous influence on society down through its long and checkered history. Look at this conclusion of Kenneth Latourette in his seven-volume work on the history of the expansion of Christianity:
No life ever lived on this planet has been so influential in the affairs of men like the life of Jesus Christ. From that brief life and its apparent frustration has flowed a more powerful force for the triumphant waging of man’s long battle than any other ever known by the human race.
By it millions have been lifted from illiteracy and ignorance and have been placed upon the road of growing intellectual freedom and control over the physical environment. It has done more to allay the physical ills of disease and famine than any other impulse known to man. It’s emancipated millions from chattel slavery and millions of others from addiction to vice. It has protected tens of millions in exploitation by their fellows. It’s been the most fruitful source of movement to lessen the horrors of war and to put the relations of men and nations on the basis of justice and of peace.
In other words, can Christianity change society? Of course. We know it can because it already has.