Just as the anthropology of Augustine has a human being searching for the Will of God in order to be most true to our human nature, so likewise does Augustine see the concept of justice as defined in terms of that which is desired of us by the Will of God.

Augustine could not have imagined an anthropology that did not give God a primary and central influence, so too he would not envision a true justice as lacking the same essential element. Discussion by Augustine about the constituents of justice and of a just society and of are found in his great work, the City of God. In Book XIX he examines the idea of justicia (justice) in De re republica ("About the Republic") by his favourite ancient Latin author, Cicero.

Augustine then quotes Habakkuk from the Bible. Habakkuk links the just person (iustus) to the justice (iustitia) due God "who rules an obedient city according to his grace." The Bible states that the just person lives by Christian faith. Augustine added that this has to be a Faith that is active in Christian love, the love by which one loves God alone and neighbour as oneself. For Augustine, justice can be well described as the respecting of God by following the law of love of the Bible – "Love God, and your neighbour just as you love yourself."

The just person par excellence is one whose Faith is demonstrated by caritas – love of God and love of neighbour. Even so, in City of God the relationship of people to justice is only secondary because, for Augustine, justice is primarily about God. In other words, wherever God does not receive his due there can be no justice. For Augustine, justice begins and ends with religious devotion, the love and adoration of God. From start to finish the approach of Augustine is theological: Justice has to do with knowing and loving God.

As defined by Augustine, justice is not fully attainable by a human being while still on earth. This should not be a surprise, because if justice is perfection, then only the perfect being — God, not humans — would have perfect access to it. A human person Man can only gain access when he or she becomes one with God, a situation entertained by the Christian religion only after death. "Life, therefore, will only be truly happy when it is eternal." For a person to be just (or, more correctly, to pursue justice), he or she must deny the love of self that is part of human nature, and actively draw towards a love of God. But, because of sin, human beings are incapable of knowing and loving God unless they accept the grace of God.

And Augustine argued that God is knowable only through the mystery of the coming of Christ to the earth. Consequently there can be no justice without Christ. Because Christ is the only one who lived without sin, he is the only truly just man (iustus), and thus the exemplar and measure of justice. For Augustine, justice was not restricted only to his philosophical reflection and spiritual meditation. As a bishop he fulfilled the role of magistrate himself, and had to dispense justice in the public court. He responded to concrete problems faced by magistrates and judges. Again and again Augustine shows that moral reasoning must be transformed by the deeper wisdom offered by Christ and the Bible.

In a case involving rebels who had repented of their involvement in an uprising, Augustine urged clemency, not only for the sake of mercy but because of justice. In those days, repentance involved a lengthy process regime of public penitence. After these rebels had in their penitence asked for public forgiveness for their error according to the ascribed public process, Augustine held that it was right and just that clemency be shown. As biblical support for such reasoning, Augustine referred to the decision of Christ regarding the woman caught in adultery, as described in the Gospel of John. Rather than convict or punish her, Jesus simply told her to go away and sin no more. If Jesus personified justice perfectly, what was his example there indicating? 

The classical pagan definition of justice was to "render to each his due." Augustine did not dispute this, but as a Christian theologian said that it had to be understood in light of the biblical precept, "Let no one owe anything except to love one another."

Augustine appreciated that for a pre-Christian like Cicero there had been only one society, the earthly society, the de re publica. But by baptism Augustine belonged to another society as well, where the "Love one another" of caritas (Christian love) was the guiding star. For Augustine, therefore, justice needed to be infused by and defined by caritas (love), as indicated above. The view of Augustine about justice, therefore, is deeply religious, and almost ascetic. It comes from a view of human nature - a Christian anthropology - in which there is not much optimism present.  

Links

Augustine and justice. According to Augustine, justice is one of the four main forms of loving. By Sr Mary T. Clarke RCSJ, a scholarly article of eight pages. PDF file.
http://www.patristique.org/sites/patristique.org/IMG/pdf/63_ix_1_2_05.pdf   

“An unjust law is no law at all.” A short summary of Augustine about Justice.
http://library.law.harvard.edu/justicequotes/explore-the-room/west 

Augustine’s Political and Social Philosophy by J. Mark Mattox in the Internet Encyclopedia of Philosophy
http://www.iep.utm.edu/aug-poso  

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