You state that your book is based on one fundamental assumption about human nature: people respond to incentives. Which is another way of saying that people are basically selfish. Take someone like Jesus Christ. What was his “incentive” to go on the cross?
It is important to define our terms. There is a difference between being selfish and having self-interest. While some incentives can be defined as selfish, people also have a necessary interest in their well-being. Food, shelter, basic care, the desire to love and be loved, and even acts such as deciding to get out of bed in the morning are good and necessary acts that are not inherently selfish. Some self-interest, then, is good and necessary for life. God created us in His image and for that reason we should have at least some interest in properly loving ourselves. Selfishness, however, is a corruption of good and necessary self-interest.
In the context of capitalism, which functions through people acting according to self-interest (remember, not necessarily greedy selfishness), people may at times choose to sacrifice for others. Since self-interest is not necessarily selfishness, then we may choose to act in the interest of others by sacrificing something of ourselves for the good of another. This is possible in part because doing good for others does not imply that we are harming ourselves by participating in that goodwill.
Unlike us, Christ’s motives were never impure or selfish. So, what incentive did Jesus Christ have to go to the cross? If we accept that not all self-interest is selfishness, then we can understand that Christ did not die because of self-centered motives but rather as a sacrifice for others. The beauty of properly ordered self-interest is that we can love ourselves while being free to serve others sacrificially. Praise be to God for the ultimate sacrifice, Jesus Christ.