Regarding the Assumption of Mary, the Catholic Encyclopedia reports:
If we consult genuine writings in the East, it is mentioned in the sermons of St. Andrew of Crete, St. John Damascene, St. Modestus of Jerusalem and others. In the West, St. Gregory of Tours (De gloria mart., I, iv) mentions it first. [...] St. John of Damascus (P.G., I, 96) thus formulates the tradition of the Church of Jerusalem:
St. Juvenal, Bishop of Jerusalem, at the Council of Chalcedon (451), made known to the Emperor Marcian and Pulcheria, who wished to possess the body of the Mother of God, that Mary died in the presence of all the Apostles, but that her tomb, when opened, upon the request of St. Thomas, was found empty; wherefrom the Apostles concluded that the body was taken up to heaven.
Today, the belief in the corporeal assumption of Mary is universal in the East and in the West
http://www.newadvent.org/cathen/02006b.htm
The Church has always considered the truth of its doctrine to be infallibly guarded by Paraclete, as promised in scripture. Naturally, Church doctrine officially defined from the Chair of Peter enjoys the same protection.
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