Belief that Mary, at the end of her earthly life, was taken up body and soul into heaven rather than left to await bodily resurrection like other believers had circulated in the church since at least the early centuries, marked liturgically by a major feast kept since antiquity, but like the Immaculate Conception before it, it had never been formally defined as a required article of faith. After consulting the world's bishops by letter, as Pius IX had done a century earlier for the Immaculate Conception, and finding overwhelming support, Pius XII issued the apostolic constitution Munificentissimus Deus on 1 November 1950, declaring the Assumption a divinely revealed dogma.
The declaration is notable as the only time since the First Vatican Council formally defined papal infallibility in 1870 that a pope has invoked it explicitly to define a new dogma. Every other doctrinal statement issued since 1870, including by Pius XII himself in his many other encyclicals, falls short of this formal exercise, which makes Munificentissimus Deus a rare and deliberate use of the specific authority the 1870 council had described, applied to a belief the church held could be shown to be part of the apostolic faith even without a single, explicit scriptural proof text describing the event itself.
The timing, five years after the Second World War's end, was read by many contemporaries as a deliberate act of hope directed at a world recently devastated by mass death, affirming that a human body, Mary's own, had already been taken whole into the glory Christian teaching promised to all believers at the general resurrection still to come.