How long from now would it take for a new religion—completely new, not based on any current religion—to evolve and become widespread? The religion would be as complex as any real religion, with its different interpretations and therefore disagreements between communities. It would also have to be as common as a religion like Christianity or Buddhism is now. Would any outside influences slow down or speed up the process of this religion becoming widespread? Would it be different now compared to 2000 years ago?
There is no set time and for you as a world builder, it can be anything you want.
Take Pastafarian. A religion that believes in a Flying Spaghetti monster. Now before you say thats not a proper religion, it was accepted as a religion in the Netherlands in 2016, so it very much is a real religion.
It started in 2006, in 2011 a Pastafarian was able to take an ID photo with a Colander helmet which was considered a religious headgear and then in 2016 it became an accepted religion in the Netherlands. So there you have a religion, completely unique created in 10 years.
With the more prolifant use of the internet in this day and age you could theoretically start a religion in less than a week by spreading it via memes and social media and gaining the endorsements of famous celebrities and company figure heads.
Religion has taken a long time to evolve to its current form.
- Buddhism is based on Gautama Buddha who lived sometime around 500-400 BCE.
- Christianity is a schism from Judaism based on Jesus of Nazareth who lived around 30-1 BCE, Judaism can be further traced back to circa 500-600 BCE.
That is roughly 2500 years of development that can be attributed. Chances are that both traditions are refinements of prior thinking and practices that might stretch back through oral tradition many thousands of years further.
There are newer supposedly religious organisations such as scientology, however these are based on relatively new models of thinking and recently manufacted tradition. To suppose that these models of thought are on the same level as the older and actively practiced religions would require a rigourous model of moral thought. Which is precisely what a religion is meant to be. This does make it hard to compare religions in any sensible manner as the comparison would presubscribe you to a religion.
There are thinkers from many fields attempting to articulate a rational approach to comparison or at least a rigorous approach that does not require a subscription to a specific belief system in order to allow a reliable and hopefully beneficial comparison.
A proposition is that a religion should provide a set of beliefs that enable the believer to act in the world so as to not increase the suffering or malevolence already present, and perhaps even remove manifestations of suffering or malevolance that is without meaning, or that are not required in some meaningful way. Many of the old and active religions today can be seen as ways of reducing suffering and malevolence, although they are far from perfect and quite susceptible to corruption.
I would argue (yes, an opinion) that to be considered as an equally valid religion would require the new organisation to encourage similar (or better) reductions in suffering and malevolence, across time, while being similarly or lesser in both severity and duration of its corruptive tendencies.
As to how fast this could be achieved is similar to asking how long a piece of string is. Certainly current day thinkers can draw on these collections of thought, but that is the same as saying that the new religion is an an evolution of that thinking. That just means that a new religion takes longer to develop now, because it has already taken as long as its oldest parent to develop.
A completely novel religion requires abandoning and never referencing prior thinking. The best proxies we have for judging how long it would take to develop a complex religion are the current day old and complex religions. So at least 2500 years. Maybe it could be done faster. I would reckon that this is however the fast track, evolutionary theory would suggest that the current old religions are present because they are the best at what they do, otherwise they would have been supplanted and abandoned. There certainly exists enough abandoned religions around the world to support this hypothesis.
In this video Is Intersectionality a Religion? A group of liberal-atheist academics give a fascinating argument that it fulfils all the requirements. https://www.youtube.com/watch?v=_AvyqUOKhGA&t=4796s
The word itself was first used by scholar and civil rights advocate Kimberlé Crenshaw in 1989. While still a student studying to be a lawyer, she saw that gender and race were looked at as completely separate issues. To Crenshaw, studying them in isolation to each other made no sense. She saw that women of colour, for example, are doubly discriminated against, particularly in law.https://iwda.org.au/what-does-intersectional-feminism-actually-mean/?gclid=Cj0KCQiAm5viBRD4ARIsADGUT27hEuTYvjxbyhQ4nB7C7xC0tHnJhDI6XaicX8GQzJU6kEMUu8QGQKMaArhgEALw_wcB
If we accept the above premises then the religion of Intersectionality has spread widely in Western cultures in 30 years.
Even if we don't accept the ideas, the talk gives a very good set of criteria as to what makes a religion what it is.