**Many of the quotes by 6th century Fathers about Mary’s assumption are related directly to Gnostic writings, Transitus Beatae Mariae (pseudo Melito), the Dormition (pseudo-John). **
This Gnostic Fable did eventually creep into the Church as Peter and Paul warned against.
2Pe 2:1 - But there were also false prophets among the people, just as there will be false teachers among you.
They will secretly introduce destructive heresies, even denying the sovereign Lord who bought them–bringing swift destruction on themselves.
There was no defense against it for the first 500 years because it was unheard of until the condemned Gnostic writings appeared .
The Church never taught it as true doctrine until close to 600 AD. Popes Gelasius and Horsmidas did fight against this, but apparently the church brought it in hundreds of years later.
The Transitus (pseudo Melito) is **exclusively **about the assumption. Pope Gelasius condemned this Gnostic forgery in 490. There is not much else that is orthodox Christian truth in these forgeries.
In the sixth century a great change passed over the sentiments and the theology of the church in reference to the Theotokos—an unintended but very noticeable result of the Nestorian controversies, which in maintaining the true doctrine of the Incarnation incidentally gave strong impulse to what became the worship of Mary. In consequence of this change of sentiment, during the 6th and 7th centuries (or later).