Personal theory here, but I think Belah'dia are the descendants of the Meracydian civilization that spawned Sophia, or at the very least that civilization once had colonies on the mainland of Eorzea that the Belah'dians would later draw parts of their religion and iconography from. At the very least, we know Qarn is supposedly much older than the Belah'dian kingdom that appropriated it, and the similarities between Sophia and Azeyma suggests to me that they're historically related somehow.
Also, I've heard that a symbol seemingly related to Sephirot is present on both the temple and Meracydian ruins found on Azys Lla, so that's another connection right there.