In the study of languages, there is a clear division into the synchronic and diachronic study of languages. Synchronic study encompasses describing language as it is or was at some point, whereas diachronic study traces the changes language has gone through. Clearly, there is a relation between these.
A synchronic description benefits from a diachronic approach in that it can tell how a given configuration came about. A diachronic description requires some synchronic description as a starting point. However, diachronic methods may help figure out details in the synchronic description and vice versa - they complement each other.
What is important though, is to realize that what is true from a synchronic approach does not necessarily hold true in the diachronic approach - nor the other way around.
Example: The /v/ in wives and the /f/ in wife, were originally perceived as the same sound. These sounds have since become distinct. We cannot claim that the /f/ in cough is the same as the /v/ in cove.
We also find this with word meanings. Today, meat signifies the edible muscle tissue of various animals (but also, in various not quite literal uses, muscle, but also the significant portion, etc). However, we also find oddities: sweetmeat is maybe the most recognizable of the oddities. Meanwhile, we also know that some other Germanic languages have words that are related to meat, e.g. Swedish mat. Mat signifies food. Turns out this originally was the meaning of meat as well, but a modern English speaker speaking of meat is obviously speaking of the English meaning, not the original, ancient English meaning. The same, of course, happens in religion: if a ritual originally came about to placate the Gods and give rain, and later on became a it's-that-time-of-the-year-hey-let's-get-drunk thingy, and the original meaning has been forgotten, and the traces of the original meaning are but obvious if you happen to have historical knowledge of the process by which the meaning changed, the celebrants are no longer placating the Gods, trying to get rain, they're probably doing something else (albeit possibly just as ritualistic and mumbo-jumboey as the previous approach!)
It seems, while reading D.M. Murdock, she sometimes thinks original meaning is the main important thing there is to religions. This is strange.