Evolution of Ideas and Faiths
Reduced rarity: In primitive times, objects (fetishes) had power because of their rarity. Over time, the improvement in industrial output allowed more people access to these tools, resulting in exponentially greater capability for magic. At the same time, the mass appearance of these objects reduced the power that each individual one had, but overall the process tended heavily to increasing magical capability.
Accessibility: The invention of new tools also led to a greater variety of spellworking ways, so that the invention of pottery led to spellspinning and the invention of the brush facilitated spellwriting, and so on. The rise of these alternate spellworking ways offered people who could not work magic with any of the existing ways, another chance using the new way. The more complex and resource-intensive a way became, the less the focus was on the individual, and so the more reliable the way was for various individuals. This greatly expanded the possibilities for working magic, as for example, spellwilling could only be used by one in 10,000’s and spellwishing was even rarer. The most modern ways by contrast are readily accessible to anyone who had the requisite materials.
Scale and reliability: It also allowed for a way to compensate for the weaknesses of other ways. In many cases the weakness was the power potential of the magic - simpler ways such as spellsaying could only rarely succeed in working large-scale magic, whereas a gem or mana based method would be more successful, even though it would be more expensive to accrue the gems or mana.
Supplements: As the trappings (ie. priestly garments) and lore (ie scripture) and pomp (ie. ceremonies) and organization (ie. state religion) backing the faiths improved, the faiths became gradually stronger, allowing for more effective, more consistent, and more complex magic. (Together, these are called the ‘supplements’ of faith.) The history of society, then, is a history of ideas, and the faiths on which they are based, evolving to become more inclusive of the general populace, so that more and more people could work magic, even if it came at the cost of a need for all these trappings of faith. As these would become expensive, only a technologically well off society could afford these ‘faith supplements’ in large quantity.
Manufaction: As more people came to have a better understanding of the truth of the faith system - the fundament of reality - and its implications, new, manufacted faiths were designed to be more powerful than their predecessors, in part by intentionally doing everything they can to derogate the existing faiths (and even faiths yet to be). Emphatically exclusive monotheistic faiths were invented by ambitious persons who thus became leaders of cults whose members were, by their faith, restricted from working other magics, and thus had none else to turn to but the cult leader. When the cult leader died, and the leader’s self-derogation effect ceased, these cults’ faiths suddenly became much stronger, and the cults became religions in their own right.