Here's the basic score, from a metagame perspective.
At the Dawn, the Caladralanders were a Fire- and Heat-oriented society who worshipped the Big Daddy, Veskarthan ("Great Father"), who came from the Sky to the Earth. In many ways cognate with Lodril, in Caladran society Veskarthan is more than some peasants' fire god, but the supreme creative, progenitve, primal, masculine force of the universe.
After Veskarthan first did the naughty with the Deep Earth, the not-so-Deep Earth, and so on, he thrust upwards to rule both Sky and Earth as Caladra - which simply translates as "prime/first crater-mountain", also known as Veskarthan's Vent (as its through this which he erupts onto the world). It is from this volcano that the land and people took their name.
Some might call this a child, but Caladralanders didn't - it was simply the supreme manifestation or volcanic aspect of their god, the mountainous pore or conduit of power through which his energies were channeled; his throne, a focus of worship. The Caladralanders did not overly differentiate between mountain and deeper source, nor did they need to; they were an insular people who lived on the slopes of a god that was wholly immanent, and they didn't have to explain everyone else's myths.
Not all Caladran tribes held exactly the same view, of course. Even in the First Age there were tribal myths that focused on the feminine placation of Veskarthan and his Vent; one tribe in particular believed that the agency of feminine demigod heroines suffused the Vent with their protective 'spirit' - the daughter aspect that would later grow. In such mythology, Caladra grew to take on a feminine nature, as a placatory interface or 'stopper' on Veskarthan's plumbing.
The God Learners did not make the female take on Caladra completely up; it would have been very difficult for them to do so, given the masculine foundations that already existed. They had problems with many Caladralanders (when it becomes more commonly available, The Durengard Scrolls will shed more light on this) but in some tribes' myths saw greater potential.
Thus, under the God Learners' leadership, this Second Age cult of Caladra began to take shape. The more violent and primal aspects of the mountain, and Veskarthan himself, were foisted upon deep, distant Lodril.
[N.B. This wasn't the first time this had happened -- correspondences with entities such as Lodril were made by such as Argan Argar himself, and the Bright Empire -- again, the God Learners weren't working from scratch.]
Caladra -- "prime crater-mountain" -- came to be appreciated with regard to the more beneficent and protective aspects, which could keep a lid on "her father's" raging. Local mountain and deeper source ultimately became two different entities, not least because of the God Learner's wider knowledge of myth, and their appreciation of volcanism across the lozenge.
[N.B. the old term for Caladra was Caladril, and Caladraland was Caladrila(r); Caladra was simply the dialectal form used by the tribe whose myths the God Learners worked upon, and has since stuck.]
Of course, the God Learners went one further, and twinned this now feminised Caladra with a minor Jrusteli Earth cult - Aurelion. This Volcano Twins Cult grew, and in the old writings was held to have survived the cataclysms that struck the Middle Sea Empire, one of the few examples of a successful God Learner experiment.
Subsequent writings, not least Greg's own thoughts, mean that this is no longer the case, or at least in its entirety. After the Closing, the majority of Caladralanders went back to the old ways of appreciating volcano and source as aspects of the same masculine entity (indeed, many had retained such patterns of worship throughout the Second Age).
The Twins Cult did not die -- and indeed many speculate that it was revived by Belintar -- and the protective, feminine aspect of Caladra still remained at a tribal, totemic level in some cases. It's just not the dominant mode of worship in the Third Age, as older write-ups stated.
For the myths of most Caladralanders as of 1621, the "prime crater-mountain" has an overwhelmingly masculine correspondence; for a lesser number, this masculine correspondence is soothed, controlled and/or propitiated by a feminine energy.
Who knows? They are an artificial cult, one of the crowning achievements of the God Learner's craft, and have a troubled history ahead of them. No doubt they have more changes in store). 
