The first thing to keep in mind is that, before Augustine shows up in Canterbury in the 590s, it's not entirely clear how pagan vs. Christian England actually is. Bede tells us that the Romano-British were bad Christians, and that they failed to evangelize the Germanic newcomers to the island, and there's no reason to doubt that many of these newcomers would indeed have brought some version of a northern European paganism with them. At the same time, there's also evidence that Christianity survived in some form during this period. Gildas, writing somewhere near the Welsh Marches in the mid 6th century, uses learned Latin and assumes his audience is Christian, and Kent - where Augustine's missionaries landed - had a very close relationship with Christian Gaul on the other side of the English Channel. So while Christianity was probably only one of many faiths in Britain before the seventh century, it was probably never entirely forgotten during the 150 years between St. Germanus' visit to get the heretics in line in the mid 5th century, and Augustine's mission to Aethelberht in the 590s.

That said, after the 590s, this pluralism seems to be rapidly erroded, and Britain soon rejoined the Roman Christian world. What changed, and why did elites support this transition?

A few factors to consider:

• Kent, where Augustine landed, was in many ways more a part of Gaul than it was of England. You see this in the region's archaeology and geography. Archaeologically, a lot of the material culture of Kent looks different from the rest of England, and similar to that in Gaul. And geographically, this makes perfect sense: you can cross the English Channel at Dover in a fishing boat on a clear day (shoot, someone once swam it!), and in many ways getting to France was easier than getting to many other parts of inland Britain. As a consequence, the kings of Kent (men like Aethelberht, who were trying to bring all their potential local competitors, the other powerful strongmen in Kent, under their control) looked to kings in Gaul as strong allies. Aethelberht's wife was the daughter of one of these Christian Merovingian kings, and was herself a Christian already before Augustine's mission landed.

  Hence, when Augustine arrived, fresh from Rome, and looking to establish a monastery from which to spread the gospel across Britain, he was doing this in a kingdom whose king was closely allied to the Christian rulers in France, and whose queen was a Christian and, Bede tells us, was able to nudge her husband further in this direction.

• Christianity also offered new ways to establish yourself as a big man (or woman) on the political landscape. Many of the first Christian foundations in England were monasteries founded by the daughters of important local elites.

  In the archaeology of the seventh century, we see a lot of evidence that suggests that, as elite men began to form powerful political dynasties (the origins of the kingdoms into which England would be divided for the next three centuries, before Aethelstan united them all under Wessex's rule), their wives and daughters gained power with them. In the sixth century, women's status seems to have been closely tied to the marriage alliances they formed (or could form): women are only buried with fine jewelry when they hit puberty, and this wealth disappears as they hit menopause. Their social position was tied up in what they brought to their husbands' beds. But in the seventh century, there's a spike of rich women being buried in special, set apart graves (often, under barrows) that can be heaped full of rich jewelry. Male graves like Sutton Hoo are the most famous burials from this century, but women were actually much more likely to be buried with treasure than men after 600. It's likely that the daughters of the new political dynasties that were increasingly parcelling up the landscape into their new kingdoms were, unlike their grandmothers, freer to enjoy power and status on their own right without having to sell themselves off to a powerful warlord to secure their status.

  These women found, in the church, a powerful new avenue to solidify this newfound political power that didn't involve marriage or a man. For many years, the most important monastic houses in England - including those that contained male monks as well as female nuns - were ruled by powerful, elite women. And these women, if we trust Bede, were often very influential in convincing the men in their lives to support the new religion.

• Furthermore, by the end of the seventh century, Britain was coming increasingly in contact with the continent through newly revitalized networks of trade and exchange. As trade emporia like Ipswich and Hamwic (Southhampton) grew and prospered, Britain's elites found themselves spending a lot of time hobnobbing with Christian elites across the water. Christianity - with its network of bishops and churchmen and its ties to distant economic powers - was a part of the world that England's elites were trying to fit into, and - whether out of personal conviction, political claculation, or the natural tendency of people to acculturate to their peer groups - England's elite became increasingly plugged in to this wider network of Christian rulers.

• We should finally, however, not downplay the role of genuine belief. Politics were important, but early medieval people also earnestly believed that having the right gods on their side conveyed material and spiritual advances, and Bede gives us several examples of rulers who employed a spiritual calculus that made them decide Christianity was worth a try. The best known story is Edwin of Northumbria, who converted after one of his nobles gave an eloquent speech comparing life to the short flight of a swallow through the twin doors of a royal hall: a brief moment of light and happiness, followed by a plunge back into the dark unknown of death. If Christianity offered hope of something better than eternity in the cold ground, sign me up! But my favorite example is actually Raedwald, the East Anglian king who was probably buried in the famous ship burial at Sutton Hoo. This king, Bede tells us disapprovingly, added an image of Christ to his pantheon because who could say which god would prove most helpful? I suspect that there were more Raedwalds than Edwins in the seventh century: men who were happy to try worshiping the powerful Roman god alongside their other supranatural helpers, because you can always use another pair of spiritual helping hands!

This is by no means an exhaustive list, but they're some of the important factors at play. I suspect /u/shlin28 might have more to add about the role of Mediterranean figures and connections in pushing this change?