"Teacher, which commandment in the law is the greatest?"
He said to him, "'You shall love the Lord your God with all your heart, and with all your soul, and with all your mind.' This is the greatest and first commandment. And a second is like it: 'You shall love your neighbor as yourself.' On these two commandments hang all the law and the prophets." (Mt. 22:36-40, NRSV)
This is one of the few places in the gospels where Jesus boils down the Jewish Law and the teachings of the Hebrew prophets in such a succinct fashion; his teachings about the Kingdom of Heaven elsewhere are often done through elliptical parables, or through his miracles of healing, feeding, and casting our demons. But here Jesus is forthrightly prescribing a rule of love to undergird all our decisions and all our behavior. This rule of love is the spirit of the law, rather than the letter of the law. Whenever Jesus is asked about the relationship between what he is teaching and the Hebrew Law, he is clear that what he offers is not in conflict with those ethical understandings. "Do not think," he tells his listeners in Matthew's Sermon on the Mount, "that I have come to abolish the law or the prophets; I have come not to abolish but to fulfill." (Mt. 5:17, NRSV)
But in fulfilling the law, Jesus constantly interprets through the filter of love. When questioned in Mark 10 about divorce, for example (Hebrew men were allowed to routinely put away their wives through a bill of divorcement), Jesus said it was not God's intent that wives be sacrificed on the altar of convenience; what God wanted in this bond of marriage was a lifelong love, and if humans were capable of it, there would be no provision for divorce. In this and in other ethical matters, he typically called people to go beyond the law, to live to a higher standard of righteousness and equity than even the law called for. So it is that we find Jesus arguing that we need to reorient our appetites as well as our actions, to recognize that our wrongfully-directed desires are as dangerous as our putting them into practice.
Augustine, who had much to say about disordered desires, seized on Jesus' teaching of the two-fold commandment as the central tenet of Christian Doctrine, and throughout his writing he repeated it, as here in On Christian Doctrine: "Whoever, therefore, thinks that s/he understands the divine Scriptures or any part of them so that it does not build the double love of God and of our neighbor does not understand it at all."
In the years since, this two-fold commandment has been passed on by the Fathers and Mothers of the Church, by Catholics and Protestants. It's been seized on by those like Martin Luther who espoused salvation by grace alone, but recognized the necessity of faithful action: "I will therefore give myself as a sort of Christ, to my neighbour, as Christ has given Himself to me; and will do nothing in this life except what I see will be needful, advantageous, and wholesome for my neighbour, since by faith I abound in all good things in Christ." ("Concerning Christian Liberty")
If love for God and our neighbor represents our highest good, then our desires will be other-oriented rather than self-oriented, and, as Augustine argued, we will best display our love for God through the tangible love and service of our neighbor. Unfortunately, love is not generally our highest good. Too many American Christians identify themselves first as Americans—self-reliant, hard-working individuals who aspire to a secular version of happiness that comes through acquisition and possession—rather than as Christians—other-directed, hard-working members of something larger than themselves who seek the joy and peace that comes from faithful service and love.
CBS News reported that for voters in the just-finished South Carolina primary, even in that conservative and values-voting state, "in 2012, economic concerns trump social and religious factors." At the end of the day, enlightened self-interest still generally determines which candidates and policies we support. If our economic concerns come first, then it becomes much easier to explain why so-called values voters in South Carolina would reject a solid social conservative such as Rick Santorum for Newt Gingrich, a man who hardly seems like the champion of marriage and traditional family values.
The philosopher Thomas Hobbes wrote in Leviathan that human beings are naturally competitive, seeking their own best interests. Augustine argued similarly that given the fallen nature of humankind and our disordered desires, we need government to tell us what to do and how to treat each other. We need an institution that can "over-awe" us (to use Hobbes' phrase) and restrain our selfish impulses. Augustine echoes Hobbes in noting that since "all in the human community are driven by personal passions to pursue their private desires," the basic state of human society is "one of conflict and war where the weak are oppressed by the strong." (City of God, 18.2)