Other than in instances of dogmatically defined doctrine, the individual conscience holds sway.
Like all Christians, Catholics see the Ten Commandments found in the Hebrew Scriptures as the basic groundwork for moral action, which together with the life of Jesus provide a deep and abiding understanding for how to act with love and justice in the world. The Gospel of Matthew relates that upon being asked which commandment was most important, Jesus replied that all of the law is contained in the commandments to love God and love your neighbour (Matthew 22:36-40).
Catholics see this as going beyond the injunctions of moral law by drawing believers into a relationship with others as well as with God, and it is the foundation of the Church’s teaching on issues of social justice.
From the earliest days of the Church, Catholics have performed works of mercy to help those who most need it, but the Church’s current involvement in social justice issues really took form in 1891 with the promulgation of the papal encyclical Rerum Novarum. In it, Pope Leo XIII called for workers to be treated with dignity and respect, protected by the state from exploitation, and allowed to form unions.
It touched off a flowering of social encyclicals that have become central to the Church’s work in the world. Catholic social teaching focuses on the dignity of the person as the linchpin for all discussions of ethics, politics, and justice. It is central to Catholic calls for the fair treatment of workers, for political systems that recognize individual rights, for responsible scientific research, for an end to attacks on human life in the form of abortion and the death penalty, and many other teachings as well.