I enjoyed the first episode of Simon Sebag Montefiore's BBC series on Rome, but not so much the second. If you're in the UK it's available on iPlayer:
BBC iPlayer - Rome: A History of the Eternal City: Divine Gamble
Sebag is normally a solid historian but this is not his field, and he seems to have sought the advice of some very dodgy sources on this journey from Rome's conversion to Christianity via Charlemagne to the Medieval popes.
What Sebag in fact visited is a second century pagan Roman house under the basilica of Santi Giovanni e Paolo al Celio (all photos are from the official web site). The church is said to be on the site of the house of Saint Pammachius (died c 409), which he turned into a basilica - but a basilica is very different from a house-church, and that basilica would have been on top of the Roman house and lost due to subsequent reconstruction of it. The hospital Pammachius built at Portus has been found, but he was born into an already officially Christian empire and so there was no need for him to hide his religion in a house-church, nor were these the norm in the Rome of his day since Christianity was already formally organised.
This fresco of the Crucifixion is high Medieval, and there are no certain Early Christian depictions of it - the first certain examples start no earlier than circa AD 400 and it didn't become popular for several centuries after that (see here)
I don't have much to add to scepticism of this supposedly early body of Santa Vittoria (martyred AD 253) except that it allegedly was found in the Catacombe di Santa Priscilla and is now in the Baroque Cappella Capocaccia of Santa Maria della Vittoria in Rome. Priscilla, like many patrons of Early Christian architecture, was a woman; this article has more about them.
Don't even get me started on the sexism in his coverage of women whether the 'wicked' Marozia or even the saintly ones. Sebag for some reason only thinks Marozia is depraved, not the men - and that is a ridiculous pattern of accusations when it comes to Medieval Italy and women wielding any modicum of power. The Empress Theophanu was "depraved" according to the sources because she bathed daily and ate with a fork rather than her hands. Do we really need to keep repeating these slurs about women? I know for sure that my own private life would sound worse if examined in this misogynistic way, and I'm a nun compared to Sebag.
The home that Gregory the Great turned into a monastery before becoming pope, S. Gregorii in Clivo Scauri, would have served as a nice example of Christian-pagan continuity and interaction: a Venus Pudica signed by Menophantos was excavated there. It was excavated rather later and not by Gregory as Wikipedia claims (no 84 in Penny & Haskell), but the taste for the Antique never quite died out, as is clear from works such as this 7th century pendant with an Aphrodite Anadyomene now in Dumbarton Oaks.
Christians continued to have a complex relationship with pagan culture and history, with one Renaissance after the other, including the Carolingian Renaissance of Charlemagne, a Renaissance under Frederick II, and there are dozens of other periods when the term 'renaissance' was used to describe them given how keenly the interest in the past is attested during them.
The nature of television documentaries is to overly simplify, but it seems a pity to almost deceive the viewer by making everything so black and white to the point where it almost claimed that black is white ...
It also might have been nice to have explained that many ancient Roman monuments were turned into fortresses, particularly along the Tiber, and that the families that controlled these and the few crossings of the Tiber controlled both the city and the papacy at least until Avignon.
I have a great deal of respect for Sebag, and this is not his period, but then again nor is it mine. Despite this, a little reading on the side and some common sense - for example noting the location of the fortresses - has served me rather better.