During World War II, Daniélou served with the Air Force in 1939–1940. With the fall of France to Nazi Germany he was returned to civilian life and began doctoral studies, completing in 1942 his thesis on the spiritual doctrine of Gregory of Nyssa. He was then appointed chaplain to the female section of the École Normale Supérieure, at Sèvres. He spent most of his time on research in patristics, and became, with Henri de Lubac one of the founders of the Sources Chrétiennes book series. In 1944 he was named Professor of Early Christian History at the Institut Catholique de Paris, later becoming dean there. Beginning in the 1950s he produced several historical studies which included "The Bible and the Liturgy", "The Lord of History", and "From Shadows to Reality" that furnished background for the development of Covenantal Theology.