The more I read about capitalism, the more its moral argument fails.
First, it has never been proven that the selfishness of the individual could ever become the good of society as a whole. The only good that has ever been created has been through accident or through self-sacrifice.
Second, Adam Smith, before he wrote the Wealth of Nations, wrote a preliminary essay on what he calls "sympathy", by which he meant empathy. His notion of capitalism depended on empathy-- the seller and employer understanding the needs of the buyer and the employee and desiring to meet it. However, our capitalistic system rather has the seller creating a need rather than meeting one. And the employer is taking advantage of the employee's desperation rather than considering their needs.
The capitalistic experiement was not to create wealth for a few, but wealth for all. Thus, the capitalistic experiment has failed.
Communism is a centrally run system where, again, the top fails to empathize with the lower classes. Thus, it topples.
Part of the problem with both systems is the natural tendancy of people to consider their privilaged status as "natural" or "normal". The corallary to that position is that other's poverty status is also "natural" or "normal".
This is why the position of Jesus and Paul is that social justice is founded on a tranformation of the mind. We have to be a bit more than human to create justice. We have to be willing to see ourselves as more than just who we are right now in order to sacrifice what we have in order to obtain benefits which we cannot see.
This is the Christian basis of social justice. Self-sacrifice to those who are not privilaged on the basis of obtaining unseen benefits. Yep, it requires faith. But that's what the church is about.