Or was what he was saying complete BS?

Ding ding ding! Ladies and gentlemen, we have a winner!

So yeah, this is pretty much all wrong, although I can probably guess what kind of narrative your friend was listening to from which he drew this impression. The 'one king' from the Rhineland who is being referred to is probably either Charles Martel (not a king, but more powerful than kings), Pippin the Short (Charles' son, who eventually crowned himself king) or Charles the Great (Pippin's son, better known as Charlemagne). In simplistic historical narratives (often peddled for political purposes) these figures are credited with 'saving' Christendom from degeneration and from Muslim/pagan conquest, through nearly a century of expansionist warfare from their heartland in the Rhine and Moselle valleys; the issues at play in the forging of the 'Carolingian Empire', as historians call it, are much more complex than that, but the fact remains that Christianity was certainly not on the brink of extinction.

That is not to say that Christianity in the 8th century was exactly the same as it was in the 9th, after the consolidation of the Carolingian state, because religion was vitally important to Frankish rulers in the late 8th century. The 7th century saw a lot of religious activity which was very localised and fragmented - the enthusiasm, fervour and ability to tap into patronage networks of individual bishops and other local church leaders (e.g. abbots and missionaries) was the key to Christianity's institutional strength in an area. This led to a lot of variety in Christianity, but it should be noted that there were a lot of religious endevours which met with tremendous success, such as the monastic foundations of St Columbanus in Burgundy and Italy, St Willibrod's mission to the Frisians and St Boniface's proselyting mission to Franconia and Thuringia. (N.B. This brief summary on pre-Carolingian Christianity is about all I've got, since this stuff is a bit outside my wheelhouse. Hopefully one of our late antique/pre-Carolingian flairs might come along and add something more!)

Those 'one king in the Rhineland' guys did change the face of Christianity quite dramatically however, even if they didn't save it from extinction (because, as you rightly guessed, it didn't need to be saved!). They did so in three major ways:

• Firstly, they promoted a radically militant, expansionist brand of Christianity, which they used as one of the reasons for the conquest of neighbouring territories. Per this ideology, the Franks were the new chosen people, the new Israelites, appointed by God to spread the Christian faith by whatever means necessary. This gradually transformed from the promotion of conversion missions in the early 8th century (Charles Martel was one of St Boniface's biggest patrons) to violent wars of conversion by the end of the century, with Charlemagne's conquest-followed-by-forced-conversion of Saxony taking centre stage - although his effort to do the same in Muslim-controlled Spain failed spectacularly.

• Secondly, the rise of the Carolingians put the Roman Pope at the heart of Christendom once again, after centuries of relative insignificance. Since Pippin the Short was not initially a king (he held the office of 'mayor of the palace', sort of like a prime minister, to the figurehead Merovingian kings), he needed the legitimacy of the Papacy in order to depose the last Merovingian, Childeric III. Pope Zachary gave a ruling that along the lines of 'he who truly holds the power should be seen to hold it' - i.e. Pippin should be king if he was already acting as king. So Childeric was carted off to a monastery, Pippin was crowned king in Soissons in 751, and even crowned a second time in Paris by Pope Stephen II in 754, just to make sure that everyone got the message that it was all legit. The papacy, in return, got protection from the encroachments of the Lombard kingdom in Northern Italy, which was eventually conquered outright by Charlemagne in the 770s. The close relationship between the dynasty and the papacy continued into the next century, when Charlemagne was crowned Roman Emperor on Christmas day 800 - thereby ensuring the centrality of the papacy in European politics for the rest of the Middle Ages.

• Finally, and arguably most significantly, the Carolingian rulers, in particular Charlemagne and his descendants, incorporated Christianity, its theology and the institutional structures of the church into the very fabric of the state. The legitimacy of papal blessing and the ideology of a new Israel were not idle propaganda; the entire apparatus of Carolingian government was geared towards renovatio - the renewal of the Christian faith. The king's court became a hive of theology and learned religious discourse, supplied by an ever-growing number of schools, monastaries and churches; churchmen became some of the most powerful political players, appointed directly by the king and played vital roles in the running of the government - Alcuin, abbot of St-Martin of Tours, or later Archbishop Hincmar of Rheims, are some of the most famous due to the numerous letters and writings they left behind which allow us to see the wheeling and dealing of Carolingian politics. Finally, religious offices and institutions were the very building blocks of Carolingian society: the local bishop was an equal and a partner to the local count, together running the county court and assembly (the mallus or placitum), mustering men for the annual assembly of the king's army, and enforcing justice and order.

In short, the Carolingian kings from the mid-eighth century didn't save Christianity from extinction, because it wasn't going extinct. They did, however, transform what Christianity meant in Europe, by integrating it into the very fabric of their kingdom/empire and disseminating that view of Christianity across Western Europe.