Something that can be very hard to conceive to our modern, post-Enlightenment mindset is the sheer extent to which religion was interwoven with the medieval consciousness. Religion was less something that you chose to believe in, and more something that just was.

Early medieval Christian theology is heavily shaped by Augustine of Hippo's City of God. This seminal work posits that, while the Empire was crumbling, it was the duty of the Church to act as the spiritual successor to the Roman Empire - specifically, while former Imperial provinces split apart into a hundred different polities, cultures and languages, this vast plethora of successor states could be united through the single unifying thread of 'Christendom'.

The Church, therefore, grew powerful along two main axes: Romanitas and bureaucracy. Romanitas, or the active embracing of Roman culture, iconography, language and socio-politics, was a massively important tool to the highly symbolic nature of early medieval kingship. Embracing the Church was the easiest way to illustrate a continuity with Rome which essentially added an instant legitmacy to the rule of a monarch. Furthermore, the Church acted as a diplomatic tool which provided a common language and purpose, and easy avenue of access to build relations between kingdoms. The eventually-extensive correspondence between Offa of Mercia and Charlemagne, for example, opens with both kings saluting each other as Christian brothers already united through bonds of faith.

The other great power base for the early medieval church was in its bureaucracy. In the power vacuum after the collapse of the Roman Empire, one of the biggest challenges facing nascent polities was the breakdown of Imperial governmental structures and a widespread decline in literacy, numeracy and general bureaucracy. Without these systems in place, 'government' declined to the petty kingdoms which could be directly controlled through the physical presence of their king or warlord. The church was then perfectly suited to fill this gap: the Church had its own established heirarchies, with systems of documentation and legislation, laws, codes, taxation and record keeping, and easily-replicable cells of control. As a key part of conversions, the Church allowed itself to be used as a part of the machinery of state government - the Church provided a model upon which secular rulers could build their own easily-replicable structures of power and control, which enabled them to oversee a much wider territorial area with a much higher degree of control than previously possible.