From Pope St. John Paul II’s encyclical Fides et Ratio (1984):
With its specific character as a discipline charged with giving an account of faith (cf. 1 Pet 3:15), the concern of fundamental theology will be to justify and expound the relationship between faith and philosophical thought. Recalling the teaching of Saint Paul (cf. Rom 1:19-20), the First Vatican Council pointed to the existence of truths which are naturally, and thus philosophically, knowable; and an acceptance of God’s Revelation necessarily presupposes knowledge of these truths. In studying Revelation and its credibility, as well as the corresponding act of faith, fundamental theology should show how, in the light of the knowledge conferred by faith, there emerge certain truths which reason, from its own independent enquiry, already perceives. Revelation endows these truths with their fullest meaning, directing them towards the richness of the revealed mystery in which they find their ultimate purpose. Consider, for example, the natural knowledge of God, the possibility of distinguishing divine Revelation from other phenomena or the recognition of its credibility, the capacity of human language to speak in a true and meaningful way even of things which transcend all human experience. From all these truths, the mind is led to acknowledge the existence of a truly propaedeutic path to faith, one which can lead to the acceptance of Revelation without in any way compromising the principles and autonomy of the mind itself.90
Similarly, fundamental theology should demonstrate the profound compatibility that exists between faith and its need to find expression by way of human reason fully free to give its assent. Faith will thus be able “to show fully the path to reason in a sincere search for the truth. Although faith, a gift of God, is not based on reason, it can certainly not dispense with it. At the same time, it becomes apparent that reason needs to be reinforced by faith, in order to discover horizons it cannot reach on its own”.91
Moral theology has perhaps an even greater need of philosophy’s contribution. In the New Testament, human life is much less governed by prescriptions than in the Old Testament. Life in the Spirit leads believers to a freedom and responsibility which surpass the Law. Yet the Gospel and the Apostolic writings still set forth both general principles of Christian conduct and specific teachings and precepts. In order to apply these to the particular circumstances of individual and communal life, Christians must be able fully to engage their conscience and the power of their reason. In other words, moral theology requires a sound philosophical vision of human nature and society, as well as of the general principles of ethical decision-making.
90 “The search for the conditions in which man on his own initiative asks the first basic questions about the meaning of life, the purpose he wishes to give it and what awaits him after death constitutes the necessary preamble to fundamental theology, so that today too, faith can fully show the way to reason in a sincere search for the truth”: John Paul II, Letter to Participants in the International Congress of Fundamental Theology on the 125th Anniversary of “Dei Filius” (30 September 1995), 4: L’Osservatore Romano, 3 October 1995, 8.
— Fides et Ratio 67-68