The magisterial figure of Y Congar has dominated all the ecclesiological questions of our time for several decades. What he contributed, as an expert, to Vatican II is not sufficiently known. After a rather timid beginning (1), various circumstances pushed him to the fore: he found a place among the redactors of nearly all the council documents (2). However he never ceased to think of Vatican II as only a privileged ecclesial moment (3) and he considered the post-council period less as a crisis than as a task «in the service of truth» (4).