A very important factor in human life is the spontaneous rise of complexity. This happens in very many human systems if enough time and resources just are available. A prime example of this growing complexity is the modern western state.
A modern society is extremely more complex than preceding societies just a couple of hundreds of years ago. The difference just boggles the mind if you really compare their systems of operation. Every new generation will add new levels complexity to the systems, when they try to perfect their inner working and to make better use of the existing ideas.

Practical life has shown time after time that the basic tendency is not to simplify, but to elaborate and develop new features to the existing systems. Of course, simplification does also occur. However, it is generally drowned in the general drive towards creating more and more complexity.
Another great example of this tendency towards the rise in complexity are religions. Most modern religions have started as rather simple and straightforward affairs. There can be just a holy man, a holy book and a handful of followers that were inspired by them. A cult is typically a religion that has not had time to develop the complexity that is normally expected from major religions.

The visarjan (nimarjan) ceremony of Lord Ganesha during the Chaturthi festival. - Wikipedia

Given enough time, layer after layer of complexity is normally added to an established religion. The new generations develop further the ideas and themes that were presented in the original founding documents by the founding fathers of the religion. However, there are huge differences how this process is handled between different religions. The biggest difference just may be between Islam and Hinduism.
In Islam, the explosion of new interpretations and ideas was firmly and thoroughly stopped in a very early stage of development of religion. However in Hinduism the growth of religious ideas has continued unabated for several millennia.

The result is that Islam and Hinduism are at opposite ends of the spectrum of religious complexity. Islam is a very controlled and restricted system. In it new ideas have no value at all if they cannot be shown to be mere interpretations of the original ideas. This is the normal route for smuggling in new ideas into all religions after the stage of their initial conception.
On the other hand, the uninhibited style of development in Hinduism has produced a veritable jungle of gods, beliefs, holy texts, holy men and ideas. It is extremely difficult to remember that also Hinduism has once started small with only a few gods, few sacred men and few holy texts.

The incredible undergrowth of religious ideas that is present in modern Hinduism is a product of generations after generations who have been working on the original ideas, and who have been producing new ideas based on the original ones. Many have introduced brand new ideas into this for a religion extremely open system of ideas.
The incredible differences between religions are often a result of the basic practical policies that will inevitably govern the development of the ideology and theology of the religions. This happens after the initial inspired stage of the founding of the religion.

Some religions are very strict in allowing new material in. The new things do consist only of interpreting the old dogmas. These religions remain basically were stable and unchanging, and often also quite simple and straightforward.
However, there is always a limit for how different material can be brought in and even in Hinduism the style and the general feeling of the new stuff must match exactly the style and the general feeling of the older layers for the new things to be accepted. If everything would be permitted the whole thing would just explode.

It is extremely easy to forget that extremely intricate and complex systems of thought and theology can be build on founding documents that are still just simple works of human imagination. When one takes the ideas, characters and stories of the founding document in their face value, it is extremely easy to add new side-characters, endless stream of interpretations of ideas and new stories that are based on the old ones.
In fact, a religious belief-system is very often like an onion. There is a layer after layer of accumulated ideas. However, at the core there is nothing when you peel the last layer away.

The Lord of the Rings: The Fellowship of the Ring is a 2001 fantasy film directed by Peter Jackson based on the first volume of J. R. R. Tolkien's The Lord of the Rings (1954–1955)

One could build a veritable pantheon of gods and an extremely impressive arrays of stories by building on the themes that are found in the fine ‘The Lord of The Rings’ by J.R.R. Tolkien, also. When hundreds of good and dedicated writers would take themselves to the task of extrapolating the themes, stories and figures found in this fine book, there would be in no time be in existence a wonderful and endless gallery of fine-sounding ideas, themes and characters.
If these writers would constantly add slight variations of the central themes and ideas produced by their own imagination, there would soon be an extremely impressive mythology in existence. This rise in complexity can be quite automatic, and it does not require any central planning. It just happens as long as it is allowed. Humans just are extremely creative creatures, if they just are allowed to be that.

There is a theory according to which the writers who take part in this kind creative process in the founding stage of a religion are not deliberately lying when they create their things. They can be self-deluded to such an extent that they really think that a deity is speaking through them when they let their creativity loose.
A real creative process simply is a situation where a person is not wholly in control of his conscious mind. The sub-consciousness just does produce material for the consciousness to use. So, the holy books are not normally based on just lies (even if the real nature of the book of Mormon or the holy books of Scientology is very suspect, to say the least). However, they are products an extremely strong will to believe that can lead to extreme forms of self-delusion.

The people who build religions based on these books do need much lesser amounts of faith and self-delusion. They can rely on the idea of divine authenticity of these documents. Extremely complex and intricate systems of belief are then built based on these founding documents.
The only problem is that they are after all always just human ideologies that are built on human ideas the writers of the founding documents and of the of founding fathers of the religion, even if their complexity as belief-systems has risen century after century.

Of course, even this needs not be a problem. This is true, if the followers of all different religions can accept the fact they are just followers of different human traditions and ideologies. Sadly this seems to be quite impossible in practice. Then the religious ideas would be judged as all other human ideas on their own merits only. They would be judged on the basis of the good things for the individual and for the society that they can bring with them.
This is seen as a direct threat among the leaders of most religions. All too often they just do not base the marketing of their core message in the true qualities and usefulness of it. Instead, they use the idea of these texts being the Only True Word of a god. Practice has just shown the latter to be a much, much easier thing to sell.

(This piece was completely refurbished on 3rd of January, 2013)