Awareness of the Christian past of Rome and the spiritual resources that lay almost untapped within it and especially underneath it had been growing since the time of Paul I (757-767). He and his successors over the next hundred years moved bodies of Christians buried in the catacombs outside the walls into the city for re-interment in its growing number of churches.
The belief that those buried in the catacombs were all victims of the persecutions of the second and third centuries grew as the veneration of the relics of martyrs became increasingly popular. As we have seen, the number of early popes believed to have been martyred expanded significantly between the fourth and sixth centuries. In many cases, no record existed of the life and heroic death of the presumed saint, and legends had to be invented to provide these bones with an appropriate past.
Roger Collins, “Keepers of the Keys of Heaven: A History of the Papacy” (p. 156). Basic Books. Kindle Edition.