From Pope St. John Paul II’s encyclical Fides et Ratio (1984):
Philosophy presents another stance worth noting when theology itself calls upon it. Theology in fact has always needed and still needs philosophy’s contribution. As a work of critical reason in the light of faith, theology presupposes and requires in all its research a reason formed and educated to concept and argument. Moreover, theology needs philosophy as a partner in dialogue in order to confirm the intelligibility and universal truth of its claims. It was not by accident that the Fathers of the Church and the Medieval theologians adopted non-Christian philosophies. This historical fact confirms the value of philosophy’s autonomy, which remains unimpaired when theology calls upon it; but it shows as well the profound transformations which philosophy itself must undergo.
It was because of its noble and indispensable contribution that, from the Patristic period onwards, philosophy was called the ancilla theologiæ. The title was not intended to indicate philosophy’s servile submission or purely functional role with regard to theology. Rather, it was used in the sense in which Aristotle had spoken of the experimental sciences as “ancillary” to “prima philosophia”. The term can scarcely be used today, given the principle of autonomy to which we have referred, but it has served throughout history to indicate the necessity of the link between the two sciences and the impossibility of their separation.
Were theologians to refuse the help of philosophy, they would run the risk of doing philosophy unwittingly and locking themselves within thought-structures poorly adapted to the understanding of faith. Were philosophers, for their part, to shun theology completely, they would be forced to master on their own the contents of Christian faith, as has been the case with some modern philosophers. Either way, the grounding principles of autonomy which every science rightly wants guaranteed would be seriously threatened.
When it adopts this stance, philosophy, like theology, comes more directly under the authority of the Magisterium and its discernment, because of the implications it has for the understanding of Revelation, as I have already explained. The truths of faith make certain demands which philosophy must respect whenever it engages theology.
— Fides et Ratio 77