Twentieth century Harvard philosopher John Rawls defined justice as fairness. Two principles undergird that proposition. The first is the principle of greatest equal liberty. In choosing, a person must decide as if in the original position, that is, from a situationally appropriate starting point with a veil of ignorance obscuring the position s/he eventually occupies. For example, in debates about slavery, if one did not know in advance whether one was to be a slave or slave owner, nobody (except maybe a masochist!) would argue that slavery was just. Rawls contended, correctly I believe, that most people who engage in this type of analysis would try to distribute rights and freedoms such that regardless of what position they occupy they are not disadvantaged when they compare themselves to other people. The result of not disadvantaging anyone is that all receive, as much as possible, approximately equal or fair benefits.
Rawls' second principle, known as the difference principle, stipulates that social and economic inequalities must satisfy two conditions: "first, they are to be attached to offices and positions open to all under conditions of fair equality of opportunity; and second, they are to be to the greatest benefit of the least-advantaged members of society." He identified five types of primary goods (what free and equal persons need as citizens): basic rights and liberties; freedom of movement and free choice of occupation; powers and prerogatives of offices and positions of authority and responsibility; income and wealth; and social bases of self-respect. (Rawls, John. A Theory of Justice, rev. (Cambridge, MA: Harvard, 1999) and Justice as Fairness, edited by Erin Kelly (Cambridge, MA: Harvard, 2001))
Justice as fairness offers a practical framework for determining just actions, policies, and programs in a wide variety of settings. Rawls presumes a democratic context, which fits both many churches (excepting authoritarian ones such as the Roman Catholic Church) and secular society.
Although he wrote from a secular perspective, nothing about his concept of justice as fairness precludes a person of faith from discerning God's spirit guiding people as they attempt to decide what is just. Sermons and church teachings about justice might receive a better hearing if the authors strived to be explicit why they declare something just or unjust using Rawls' two principles of justice.
Unfortunately, little agreement exists about what specifically constitutes the set of basic human rights or primary goods. Are these rights only political (free speech, free exercise of religion, right to due process of law, etc.)? Or, do these rights also include the right to life and, if so, what does that include (the right to be safe in one's own person or also the right to water, food, shelter, and perhaps healthcare)? Once delineated, how does a society most justly resolve conflicts in rights between one person/group and another?