Why do they put any emphasis on Mary?
I'm doing research on Roman culture for a book I'm hoping to write. It's been an eye-opener. There are lots of things going on in Roman culture that revolved around the goddess religions. They had for example a female deity called Regina Caeli, which means "Queen of Heaven." Virgo was another, which by name means "The Virgin." The Vestal Virgins were priestesses of the female deity Vesta, who were sworn to an oath of celibacy, and had been practicing this for about three quarters of a millennium before Christianity appeared. The structure of the college of cardinals corresponds nicely to the organization of the Roman senate, and Caesar could speak on occasion with the authority of a god on earth, as the pope also claims, when speaking "From the Chair," Ex Cathedra. I could go on and on. The parallels seem endless.
The point is, this is the culture into which Roman Christianity was grafted, and it was pervasive. Every last person who became a Christian in Rome, unless they were imported from other lands and cultures, had all these templates for religion (and many, many more I haven't even mentioned) baked into their worldview. The interesting thing is that during the first two centuries, Christianity in Rome was largely held by the "riff raff," the social outcasts, who were largely imports from other lands, many slaves captured and brought in to supply Rome with an inexpensive and disposable workforce. It was exactly as Paul said, not many wise, not many noble.
And for those first two centuries, while Christianity remained largely a curious cult not popular with the upper class, the assemblies remained diverse, without any discernible central leadership, and little impacted by the pagan influences surrounding them. The later developed lists purporting to show a continuity of popes during that early period has proved to be impossible to support from primary sources. See for example Peter Lampe's landmark work, "From Paul to Valentinus: Christians at Rome in the First Two Centuries:"
http://triablogue.blogspot.com/2014/01/an-extended-review-of-peter-lampes-from.html
According to Lampe, the early congregations were diverse and decentralized, with no single "Bishop of Rome." What did happen is that toward the end of the Second Century, power began to concentrate in those individuals in Rome who gathered the ecumenical funding for charitable projects outside of Rome, and it is from these late roots that a more traceable papacy would ultimately emerge. Their claim to the "Throne of Peter" was retroactive, but not based on demonstrable history.
Then, when the tide finally began to turn at the beginning of the Third Century, Christianity, again according to primary sources dug up by Lampe and others, began to be more accepted in the upper classes of Rome, where full on cultural Romans set themselves to reconcile this curious new cult with the pagan religious atmosphere that dominated Rome at the time. It is easy to see how the Christian stories could supply them with a rich supply of material that to them seemed an honest correspondence with the pagan deities, practices, and ecclesiastical structures in which they had been immersed for centuries. They were going to "adopt" Christianity and dress it up in Roman attire, because that was what they knew. In hindsight, it would be a hard error to avoid, as we all have a tendency to see divine truth through our own culturally tainted lenses.
Peace,
SR