The term of Athanasius contra mundum refers to the fact there was a time where the Arains (i.e. ancient Jehovah's Witnesses) almost politically controlled everything. They had the Emperor's ear. Even the Bishop of Rome, after being tortured, acceded to their faction. For a small period of time their faction actually politically had power.
However, Christianity was not an elite religion. Controlling the political machinations of the Church as not going to convince people, who have been CHristian for generations, to dump a Trinitarian understanding that clearly is found in Scripture and all early historical works of the Church (1 Clement and the Didache for example.)
So, I honestly find the OP almost ASB. It presumes that Trinitarianism wasn't already heavily entrenched, and well understood, orthodoxy. Further, it doesn't help Arianism that it has no Scriptural or traditional support.
However, I won't say it is completely impossible. In the middle ages, much of the practice and doctrines of the Church no longer shared much resemblance to the first few centuries. What is needed is for earlier Arianism not to exist, or to be a teeny-tiny heresy that is crushed and almost completely forgotten (i.e. like 200 different odd ball heresies that Irenaeus writes about in Against Heresies.)
Heresies have historically strengthened the early Church, because it resulted in the creation of creeds and formalized doctrines so that they were crystal clear. If the doctrine of the Trinity is simply not considered that big a deal and the Church instead gets rocked by other heresies that take up its formative years (perhaps Predestination, Syncretism, and perhaps more fight over Books of the Canon) then all it takes is a few heavy hitters to dabble with Arianism with a mix of Aristotelian philosophy.
It becomes to the next big thing in European universities. Many Bishops are Arians, but they get along with Trinitarians. Eventually, the Arians outnumber the Trinitarians in the west. The Eastern Church view this issue, along with disagreements on what the West believes to be the correct canon, and the date of Easter, does not agree with where the Bishop of Rome is taking things. The result, is that they excommunicate each other.
As a result, both Churches create creeds, one Trinitarian and the other Arian.
Being that the RCC dominates the nations that will in the future expand and become global powers, Arianism grows into the most popular branch of Christianity. Over political divisions, and the conviction that the Pope is too slavish to Scripture and not the Magesterium, a Protestant movement breaks out accusing the RCC of being Sola Scriptura. THey create their own Confessions where they view Sola Ecclesia the sole basis of religious authority. THe problem is, being they are schismatics, each nation-state essentially creates their own Churches while the RCC in the Council of Trent digs its feet in and pronounces that the Scripture is the sole authority for religious matters, and the Pope is the teacher that God ordains for the Church to follow. Both Western Churches are Arian in flavor.