As with the last part, we’re going to delve into a topic that may be a bit controversial. This time around, it’s the other half of church and state: religion.
For some languages, the whole subject is unnecessary. Quite a few, even among fictional conlangs, won’t need too many words for religious concepts. Auxiliary languages can likely get by with borrowing the needed terminology. And a far-future sci-fi setting might consider religion to belong to an earlier era.
On the other hand, even if the hypothetical speakers of your conlang don’t need to talk about their religion, that doesn’t mean they won’t want to talk about any religion. So it helps to have a bit of vocabulary specifically tied to the subject.
Gotta have faith
Religion and spirituality, in some form, have been around since the earliest days of humanity. Even if it’s nothing more than simple ceremonial burial, you can find evidence of the practice from the Stone Age, and some of our oldest human creations are religious in nature. It stands to reason, then, that a few basic ideas are going to be universal. The specifics might be wholly different even between two neighboring cultures, but they’ll both likely have some common ground in the fundamentals.
According to those who study the field (I don’t), religion of any kind probably started when someone first asked, “Why?” Why is the world like it is? Why do the seasons change? Why do people die? Maybe they begin as simple answers to those questions and more, or a shared set of stories, myths, and legends that only increase in popularity as they are told and retold over the generations.
This bare summary already gives us fertile ground for linguistic roots. The concepts most common to all religions are very likely going to be represented by native terms: faith, prayer, blessing and cursing, gods (or a monotheistic God, such as the case may be), an afterlife. Depending on the culture, you can also add in those placed in charge of religious matters, whether priests, shamans, or something else entirely. The ceremonies, rituals, and rites will also be in this field; they’ll likely be too specific to translate directly, but the words describing them won’t be.
As the folklore surrounding a religion grows, it necessarily gains a bit of verbal cruft. Even in Western Christianity, you’ve got quite a lot of vocabulary, from saints to bishops to crusades. (Note that many terms associated with Christianity, like “crusade”, tend to be related to “cross” or its analogues in Latin, Greek, and the Romance languages. That’s certainly not a requirement, but more of a historical quirk.)
Not only does a growing religion gain more words, but it also spreads across the lexical space, as it envelops closely related fields. Western faiths might all be monotheistic, but they each have a collection of supernatural beings, including (to use Christian-specific terms) angels, devils, demons, and ghosts.
This is where the twin forms of borrowing come into play. First, a highly organized religion will be able to spread its message far and wide, sending its specific terms to new places on the lips of its priests. So many English religious words come from Latin and Greek for this very reason. Similarly, Arabic loans related to Islam pop up everywhere from the western coast of Africa to the farthest reaches of Indonesia.
The second bit of borrowing comes when a new religion overtakes an old one. Here, it’s not so much that new words are borrowed, but the old ones may be reinterpreted, then spread in their new connotations. An example might be English ghost, which seems to have spent the last thousand years or so cycling between referring to a malevolent supernatural entity, the haunting spirit of the deceased, or even a kind of supernatural essence (as in the word spirit, itself a Latin loan). Fairies got their own bit of folk reinterpretation, while possibly-wise daemons became always-evil demons.
No matter what your conlang’s speakers believe, they’ll have a number of words specifically for their religion. The native terms will be made for that. If, along the way, the people were converted to some other faith, then they’ll likely take it in one of those two ways. Either they’ll import the words they need (spirit, bible, angel) from the “official” language of their church, or they’ll take some of their own and remake them to fit the new worldview: ghost, holy. Coinages tend to come about for new ways of thinking about the religion, and even then they aren’t made from whole cloth.