Pope John Paul II, in his second encyclical since his pontificate began two years ago, has called on nations as well as individuals to abandon the "eye for an eye and a tooth for a tooth" philosophy of the Old Testament and instead let mercy and justice guide relationships.

The 30,000-word discourse, entitled "On The Mercy of God," is a closely reasoned theological treatise that befits the work of the scholar and former university professor of philosophy. As did his first encyclical, this one seeks to link the central doctrines of the church to the problems of the modern world. His concern embraces all mankind and is not limited to church members. w

The encyclical portrays mercy as most dramatically demonstrated by Christ's willingness to be crucified for the salvation of humankind. The pontiff makes only brief references to the need for mercy, as well as justice, in national and international relations.

"In the modern world the sense of justice has been reawakening on a vast scale," the pope says -- a trend that he believes "gives proof of the ethical character of the tensions and struggles pervading the world."

At the same time, he continues, "very often programs which start from the ideas of justice and which ought to assist its fulfilment among individuals, groups and human societies, in practice suffer from distortions. Although they continue to appeal to the idea of justice, nevertheless experience shows that other negative forces have gained the upper hand over justice, such as spite, hatred and even cruelty."

Although the pope makes no practical application of his call for mercy as a national policy, his careful description of the need to go beyond justice and act with mercy forms an important theological base for future church documents in a variety of areas.

It would add significant weight, for instance, to a statement condemning capital punishment that was adopted last month by the American Catholic hierarchy. It also could influence future church debate on war, nuclear weapons, conscientious objection and related subjects.

"Modern man often anxiously wonders about the solution to the terrible tensions which have built up in the world and which entangle humanity," the encyclical says.

"And if at times he lacks the courage to utter the word 'mercy,' or if in his conscience empty of religious content he does not find the equivalent, so much greater is the need for the church to utter this word, not only in her own name but also in the name of all the men and women of our time."

The pope defines mercy as "an indispensable dimension of love; it is as it were love's second name and, at the same time, the specific manner in which love is revealed and effected" in the world.

Often, he says, in a paragraph that seems to allude to Marxist rhetoric of class struggle, "in the name of an alleged justice (for example, historical justice or class justice) the neighbor is sometimes destroyed, killed, deprived of liberty or stripped of fundamental human rights."

Such historical evidence "demonstrates that justice alone is not enough . . . if that deeper power, which is love, is not allowed to shape human life in its various dimensions."

"On the Mercy of God" builds on the pope's first encyclical, "The Redemption of Man," issued in March 1978, and on several documents of the Second Vatican Council. Of the council, which ended 15 years ago, the pope says that it is "not permissible for [the church], for any reason, to withdraw into herself" and abandon "the great task of implementing" the reforms mandated by the council.

One section of the encyclical, describing the moral malaise of modern man worldwide, is strikingly reminiscent of recent American political rhetoric: ". . . One cannot fail to be worried by the decline of many fundamental values, which constitute an unquestionable good not only for Christian morality but simply for human morality, for moral culture. These values include respect for human life from the moment of conception, respect for marriage in its indissoluble unity, and respect for the stability of the family."