Carolingian Christianity relied heavily on clerics in order to hold the religious establishment together; because literacy had dwindled to such a stark low, and because the Carolingian authority demanded such a heavy hand over the regulation of religious life—clerics were responsible for drawing up charts, charters, and religious documents, including their distribution throughout the territories, not to mention the religious establishment was responsible for vetoing papal elections. So, control over the clergy, and to an additional extent the literacy of them, remained an important issue.
In 824, Lewis the Pious made an agreement with the Pope where only when notified in advance, could a pope receive confirmation. The Carolingians were also able to decisively intervene in issues of what constituted Christian orthodoxy. But what their most important theological power was likely within the realm of iconoclasm (the issue which had driven the papacy from the Byzantine empire in the early eighth century); in 787, after negotiations in Nicaea had seemingly drawn the papacy and the Byzantines closer together, and an irate Charlemagne upset over the exclusion of the Franks, Charlemagne summons his own council in Frankfort at 794. This council condemned what the council of Nicaea had drawn upon, badly misunderstanding the position (it was thought that the council permitted the worship of images, which was not true). Indirectly, the pope is even condemned before being forced to tow the Carolingian line on iconoclasm.
In addition to this control over religious laws and dogma, the Carolingians supported missionary work in pagan territories to the east of them. Logistical support and even protection was provided by the Carolingians to those missionaries who wanted to go and promote the Christian faith; such was Carolingians policy for close to a couple centuries. Many of these missionaries were Germanic in origin and so were recruited to return back to ‘the old country’ and convert those who had not already converted.
Unfortunately for Carolingian sponsored missionaries, the efforts to convert the Saxons did not treat much water. Saxon kings were hesitant to convert if they were not able to see their ancestors in the afterlife while the common person felt the views incompatible with their own lives. It would not be until military conquest, and around Charlemagne’s time, that Germanic conversion began to happen en mass.
Another Carolingian effort in regards to Christianity was to encourage church reforms. Specifically, many Frankish bishops and clergy came from the aristocrats and found it difficult to give up their lifestyle—hunting, fighting (in war), and having families. The Parish system would be created in an effort to balance out these discrepancies. The Carolingian efforts were to regularize the system to the point where everyone had access to a parish and a priest; so in 765, Pippen the Short ordered that each diocese, should be divided up into parishes—one per village. Each parish is to have its own church and priest. Though, in practice, it is difficult to build and staff all these churches, the over-all result is that the number of churches increases; in order to pay for all these new churches, the Carolingians instigated the tithe, a mandatory payment which Christians must make to their church which amounts to one-tenth of their regular revenue.
But the Carolingians were also interested in monastic reform. After the Anglo-Saxon imposed discipline wavered, there proliferated a series of different monastery routines (varying amounts of prayers, not having to fast as often, etc.). A Goth by the name of Benedict would lead to substantial reform after joining a monastery and not finding it up to his personal standards; leaving said monastery, he leaves to live life as a hermit, but like Anthony before him, people follow and he must build a monastery. Eventually, the emperor attempts to extrapolate Benedict’s reforms onto the whole imperial system. Though this effort is only difficultly applied and sporadic in application, it nonetheless succeeds in raising the overall level.