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Cardinal Joseph Ratzinger is one of the theologians who has marked the life of the Church in recent years. Professor of theology (in Tubingue and Ratisbonne), then archbishop of Munich, he has often been the subject of discussion since he was named at the head of the Congregation for the Doctrine of Faith, taking position especially on the Catechism, Liberation Theology and the post-Council Church ("Entretiens sur la foi"). Is there a link - or a break - between the Cardinal's magisterial positions and his former work as a theologian and cofounder of the review "Concilium" he whose book "La Foi chrétienne hier et aujourd'hui" (1969, reprinted in 1985) was hailed for its equilibrium and open spirit? This book answers these questions by firstly presenting the essential points of the theological thinking of Joseph Ratzinger, theologian and cardinal (part one) then confronting them with the major evolutions and innovating currents of contemporary theology (part two). This comparison forms a dialogue between Christianity and modernity.