The Theological and Philosophical Origins of the Liturgical Movement

Ratzinger mentions Odo Casel as the source of the mystery theology of the liturgical movement of the 1920's and 1930's.

"[Gottlieb] Söhngen]...ventured out with great competence into the mystery theology founded by Odo Casel, the Benedictine monk from Maria Laach. This theology had grown directly out of the liturgical movement, and its very existence posed with new acuteness the basic question concerning the relationship between rationality and mystery, the questions concerning the place of the Platonic and the philosophical in Christianity, and, indeed, the question about the essence of Christianity."
Milestones, pp. 55-56.

There is a lengthy article on Odo Casel's mystery theology here.

Casel's had a doctorate in philosophy from Bonn (1919) with the thesis title: De philosophorum graecorum silentio mistico. He also had a theological doctorate from the Benedictine Athenaeum of Sant' Anselmo in Rome with a thesis on the eucharistic theology of Justin Martyr.