The Second Vatican Council here teaches that peace is not just an absence of war but is a fruit of justice and love. It can never be established by law alone but depends on virtue that grows and is maintained by constant effort in response to the gift of God. This teaching is found in the Pastoral Constitution on the Church in the Modern World of Vaticn II, Gaudium et Spes.
Peace is not the mere absence of war or the simple maintenance of a balance of power between forces, nor can it be imposed at the dictate of absolute power. It is called, rightly and properly, a work of justice. It is the product of order, the order implanted in human society by its divine founder, to be realized in practice as men hunger and thirst for ever more perfect justice.
The common good of the human race is subject to the eternal law as its primary principle, but its requirements in practice keep changing with the passage of time. The result is that peace is never established finally and for ever; the building up of peace has to go on all the time. Again, the human will is weak and wounded by sin; the search for peace therefore demands from each individual constant control of the passions, and from legitimate authority untiring vigilance.
Even this is not enough. Peace here on earth cannot be maintained unless the good of the human person is safeguarded, and men are willing to trust each other and share their riches of spirit and talent. If peace is to be established it is absolutely necessary to have a firm determination to respect other persons and peoples and their dignity, and to be zealous in the practice of brotherhood. Peace is therefore the fruit also of love; love goes beyond what justice can achieve. Peace on earth, born of love for one’s neighbor, is the sign and the effect of the peace of Christ that flows from God the Father. In his own person the incarnate Son, the Prince of Peace, reconciled all men to God through his death on the cross. In his human nature he destroyed hatred and restored unity to all mankind in one people and one body. Raised on high by the resurrection, he sent the Spirit of love into the hearts of men.
All Christians are thus urgently summoned to live the truth in love, and to join all true peacemakers in prayer and work for peace. Moved by the same spirit, we cannot but praise those who renounce violence in defence of rights, and have recourse to means of defense otherwise available to the less powerful as well, provided that this can be done without injury to the rights and obligations of others or of the community.
This excerpt from the Second Vatican Council’s Pastoral Constitution on the Church in the Modern World (Gaudium et Spes, 78) on peace as the fruit of justice is used in the Roman Office of Readings for the 31st Sunday in Ordinary Time.