In the light of what the encyclical Fides et ratio proposes concerning the relations between philosophy and theology, the paper explores themes involving fundamental theology, especially the relation between philosophical access to God and Biblical Revelation. The encyclical does not present the natural knowledge of God as a mere possibility of reason, but stresses its historical actuality and effectiveness, declaring its role as a necessary presupposition for the intelligibility of Revelation. A brief discussion is offered of the metaphor of the "book of natur", noting that, for the first time, the Magisterium presents creation not only as a witness or manifestation of God, but also as the first step in Revelation. The bearing of this on the relation between the God of Abraham and the God of the philosophers and the scientists is discussed. Alongside the circularity between philosophy and theology, note is taken of a circularity between philosophy and religion, showing the relevance for interreligious dialogue of openness to a universal philosophy.