Christianity wasn't really forced on people by a sword until later. The Romans became Christians gradually of their own accord, as those whom Paul and the others evangelised. By the time Christianity was prevalent in Rome and the Middle East the Pagan temples were gathering dust; Emperor Julian tried to resurrect the Pagan situation but failed. Christianity had triumphed without a hint of violence. Egypt converted without violence, for example.
In the European colonies, the same thing happened. The Celtic peoples of Gaul, Britain, Iberia etc. wanted to be Romans and thus took on Roman culture, which included Roman religion. So the Celts became Christians of their own will, trying to ape Roman custom (and many really were Romans by now, having been under Roman yolk for hundreds of years and knowing no different). Many became heretical types, such as Pelagians, but were no longer Pagans at any rate.
The Germanics who invaded, largely the Eastern Germanics (Goths, Vandals, Lombards mainly) became Arians, another kind of heretical Christian. By 476, Odoacer, an East Germanic, took the throne and became Western Roman Emperor. He was an Arian. Theodoric, another Eastern Germanic, reigned after Odoacer from 493 as 'King of Italy' and was an Arian Ostrogoth. Both coexisted with the Catholic/Orthodox (what would become so) Christians, maintained friendliness with Popes etc. and there was no idea of religious violence against them.
All of these groups, the majority of Europeans at the time, were converted voluntarily and by 500 most of Roman Europe was Christian. By 800 this included some Slavs, Saxons etc. the people who had never been under Roman dominion in the West but came under Eastern Roman dominion, mainly Slavs, and so many took to Orthodox Christianity. This was largely a muddled situation with both Latins and Greeks (West/East) trying to evangelise the Slavs to their own. So Poland and Czechia become Catholic, for instance, while Serbia, Bulgaria, Russia etc. become Orthodox. But these people were missionaries going to these places, not soldiers with weapons, and were often killed in their missions.
The Vikings were converted usually as they settled in Christian countries, namely England, Scotland and Gaul. By the first millennium, the Normans were Catholics and all the Pagan Vikings who'd settled in Britain were also Christians. When William invaded he found no Pagans, nor expected to. The Vikings converted largely to be able to have relationships with the rest of Europe. If Gaul allowed them to settle they would convert. The Vikings accepted the deal and stuck to it. This idea spread to their homeland; it was pragmatic and political, but it wasn't 'at the point of a sword'.
What Charlemagne did to the Pagan Saxons was inexcusable even in his day. The Church fathers had forbad it and the Saxons strongly resisted. This may be said to be forced conversion.
The insular Saxons, however, were converted gradually largely by Irish missionaries, but also by the missionaries sent by Augustine of Canterbury. It took about ~100 years to convert them, but by the time the Vikings were invading there were broadly no more Pagans in mainland Britain. So no forced conversions there.
The situation with the Slavs was complex and fraught with racism and imperialism, and they once deconverted back to Paganism.
#Mediaevalist.