The New Testament gives witness to the confidence of the first Christians in the ability of human reason to receive God's self-revelation and to foster its transmission through understanding, articulation, and proclamation. The announcement in the Gospel of John that "the Word became flesh" (1.14) affirms directly the mystery of the Incarnation, but indirectly it also indicates the dignity and efficaciousness of human reason (logos ), derived from and reflective of the divine Logos. The Apostle Paul does not hesitate to remind his readers that "invisible realities, God's eternal power and divinity, have become visible, recognized through the things he has made" and so people "certainly had knowledge of God" (Rom 1.20–21). In his letters he uses logic to refute his opponents (e.g., Gal ch. 2), as well as analogies drawn from history (e.g., Rom ch. 4) and experience (e.g., 1 Cor ch. 12).
The confidence of the Church in the ability of human reason in matters divine is well summarized in Vatican I's Dogmatic Constitution Dei Filius (1870). The Council teaches that "God, the source and end of all things, can be known with certainty from the things that were created, through the natural light of human reason" (DS 3004;J. Neuner and J. Dupuis, ed., The Christian Faith [New York 2001], no. 113). But the Council also affirms that "revelation is to be judged absolutely necessary … because God in His infinite goodness has ordained us to a supernatural end (DS 3005; The Christian Faith, no.114). There are, therefore, truths revealed by God not accessible to human reason (e.g., God as Triune, the mystery of the Church, etc.), which human beings accept through the gift of faith. Even in regard to these truths, however, there is a role for human reason, as Vatican I declared: "Nevertheless, if reason illumined by faith inquires in an earnest, pious and sober manner, it attains by God's grace a certain understanding of the mysteries, which is most fruitful, both from the analogy with the objects of its natural knowledge and from the connections of these mysteries with one another and with man's ultimate end" (DS 3016; The Christian Faith, no. 132).
Christians have always discovered fruitful insight into the mysteries of revelation by using analogies drawn from natural knowledge. Some have been quite simple, e.g., comparing the unity of Christians gathered in communion with Christ at the Eucharist to the grains in a loaf of bread. Some have been quite complex, e.g. St. augustine's psychological analogy of the human experience of memory, understanding, and will for insight into the mystery of the Trinity.
Throughout the centuries theologians have striven to connect the mysteries of revelation with one another in order to achieve greater understanding through such a synthesis. Since Vatican II, a particularly effective connection has been that of seeing ecclesial communion in the light of Trinitarian communion. In the Middle Ages, the most successful and enduring synthesis was the Summa theologiae of St. thomas aquinas. In the twentieth century, the most influential systematic work has been the Church Dogmatics of the Protestant theologian Karl Barth, who maintained that the best apologetic was good dogmatics.
The effort to connect the mysteries of revelation with the ultimate destiny of human beings has manifested itself in the twentieth century with the concern to show the coherence of revelation with the structures of human knowing and loving in the quest for transcendence. This has led to the reshaping of fundamental theology from a more anthropological perspective and reflects the modern "turn to the subject." Investigation into human knowing and loving have been intrinsic to the work of such theologians as Bernard lonergan, S.J., and Karl rahner, S.J.
Efforts to achieve greater understanding of the mysteries of revelation have to rely on the intellectual resources of a particular culture, especially philosophy. In Western Christianity the philosophy of St. Thomas Aquinas, derived from Aristotle but also modified by him to be more consonant with the Christian tradition, was for a long time the dominant force. As Pope john paul ii noted in his encyclical fides et ratio of 1998, "The most influential Catholic theologians of the present century, to whose thinking and research the Second Vatican Council was much indebted, were the products of [the] revival of Thomistic philosophy" (no. 57). Many theologians of the twentieth century, however, have also been influenced by other philosophical systems, e.g., existentialism, process philosophy, and phenomenology. Pope John Paul II himself insists that "the Church has no philosophy of her own nor does she canonize any one particular philosophy in preference to others" (Fides et Ratio, no. 49).
Theological reasoning, however, also has a critical function because of "the historical condition that affects the expression of revelation," as this is described in the Declaration Mysterium ecclesiae from the Congregation for the Doctrine of the Faith in 1973 (A. Flannery, ed., Vatican Council II: More Post Conciliar Documents [Northport, N.Y. 1982], pp. 433–34). The Declaration notes that dogmatic formulas are conditioned by "the expressive power of the language used at a certain point in time and in particular circumstances," by incomplete (though not false) formulation at first but fuller expression later "in a broader context of faith or human knowledge," by concentration on "solving certain questions or removing certain errors," and by traces of "the changeable conceptions of a given epoch." The Declaration goes on to define the work of theologians as "seeking to define exactly the intention of teaching proper to the various formulas, and in carrying out this work they are of considerable assistance to the living Magisterium of the Church, to which they remain subordinated."
Vatican II led to the use of other resources for theological reasoning besides philosophy. In discussing the participation of the People of God in Christ's prophetic office, the Council states that in adhering to the Christian faith the People "penetrates it more deeply through right judgement, and applies it more fully in daily life" (Lumen gentium, no. 12). This has led to greater theological reflection on the practice of faith by the members of the Church. Praxis has become a source of insight into the implications of the Gospel and the Christian tradition for teaching and action. Thus, for example, liberation theology, insofar as it is theological reflection on the plight of the poor and their Gospel-based actions to improve their condition, underlies the Church's recognition that "there is a special presence of Christ in the poor, and this requires the Church to make a preferential option for them" (Pope John Paul II, Novo Millenio ineunte, no. 49).
Theologians have also turned to the sciences—empirical and social—either engaging them as conversation partners with the Christian tradition or using them to gain deeper insight into the unfolding of the tradition. Pope John Paul II notes that "reference to the sciences is often helpful, allowing as it does a more thorough knowledge of the subject under study, but it should not mean the rejection of a typically philosophical and critical thinking that is concerned with the universal" (Fides et ratio, no. 69).
Vatican II also encouraged young Churches to "borrow from the customs, traditions, wisdom, teaching, arts, and sciences of their people everything which could be used to praise the glory of the Creator, manifest the grace of the Savior, or contribute to the right ordering of Christian life" (Ad gentes divinitus, no. 22). The Council encouraged theological investigation in each of the great socio-cultural regions so that "the facts and words revealed by God, contained in Sacred Scripture, and explained by the Fathers and Magisterium of the Church, [could be] submitted to a new examination in the light of the tradition of the universal Church" (ibid.). Pope John Paul II notes that the Church of the future "will judge herself enriched by all that comes from today's engagement with Eastern cultures and will find in this inheritance fresh cues for fruitful dialogue with the cultures which will emerge as humanity moves into the future" (Fides et ratio, no. 72).
In sum, the Church maintains a positive regard toward the role of reason in the relationship of human beings to God. Reason can come to know the existence of God by its own natural light, and it can achieve a deeper understanding of God's self-revelation through the use of philosophical systems, the sciences, praxis, and dialogue with other cultures.