Christian iconography and its visual vocabulary derive fromg both the allegorical language used by Christ himself, as well as the need for covert symbols in Christianity's underground days prior to Constantine...
It is very ancient. Sadly, many of us have forgotten how to read it.
The Church, of course, also kept a lot of poetry alive, and it was recited by the people (think of things like the Litany of Loretto, with its beautiful titles of the Virgin, many of them taken from the Song of Songs or other parts of the OT). Very often these phrases were also expressed in the churches as visual images - sculpture or murals - which everyone recognized.
We are so lacking in this vocabulary now that people have no idea what they’re looking at. I lead tours in a Cathedral that was heavily renovated in the early 1960s, although fortunately this was still a time when traditional imagery was incorporated into the decorative art in churches. I always love explaining the symbols to people. But what’s sad is that many of them nowadays really have never heard it before. We’ve lost the language.