I am not an expert on this subject, however my understanding is that St. Thomas is considered a major source of much of the story surrounding the Dormition and the Assumption. But I also believe this falls under the heady of what we might call a “pious tradition of the Church.” The Feast is kept by the the entire Church, so the Dormition itself is pretty much dogma. However the Church has never spoken with one voice on the origins of the great Feast.
typo
heady = heading
The bodily asumption is theologoumennon, though I personally have never met an Orthodox Christian who didn’t believe it. The Dormition, meaning “The Falling Asleep” or death of the Most Holy Theotokos is simply a fact.