Social justice is embedded in the guiding principles of our apostolate, and very early in my study of it as I was becoming Catholic, it was clear that the primary social justice issue was respect for life, for all other issues of social justice rest on that.
George Weigel notes the importance placed on the respect for life in the newest encyclical from Pope Benedict XVI.
An excerpt.
“In any event, there is an important theme in Caritas in Veritate that, were all Catholics to take it seriously, might have a measurable impact on the American culture wars and on the U.S. Church's internal struggle to define Catholic identity -- and that is the encyclical's insistence, repeated several times, that the life issues are social justice issues, so that Catholic social doctrine includes the Church's defense of life from conception until natural death.
“This teaching began with John Paul II's 1995 encyclical, Evangelium Vitae (The Gospel of Life), in which John Paul warned that democracies risk becoming "tyrant states" if moral wrongs are legally declared "rights." Then-Cardinal Joseph Ratzinger went a step further in his homily at the Mass for the Election of a Pope, on April 18, 2005.
“There, Ratzinger warned against a "dictatorship of relativism" in which coercive state power would be used to enforce the by-products of a culture skeptical about the human capacity to know the moral truth of anything: by-products such as abortion-on-demand, euthanasia, and "gay marriage." Now, as Benedict XVI, Ratzinger has moved the discussion further still, teaching that the defense of life is crucial to building the "human ecology" necessary to sustain just economic practices and protect the natural environment.”