I have not read Spengler, but I think this perspective is compelling: the later Platonists' systems seem at least as much about the various mystery cults as about ancient Hellenic practices, if not more so. That said, it seems like the various groups of that period—Pagan Neoplatonists, Gnostics, early Christians, et al.—all seem to be responding to something in the spiritual environment, and, per the passage sdi found, it seems as their responses had some similarity.

I suspect it's possible to overstate the Plotinus-Iamblichus divide, but that said I tend to fall more in the Iamblichus camp. He seems to always be tying the more theoretical aspects back to how they explain real lived "mystical" (for lack of a better term) experiences and ritual. (I will cop to being a simpler person, more concerned with the practical than the abstract, FWIW.) That's what I find so valuable about these writings: not that they are some kind of dogma or doctrine per se, but rather that they represent attempts of other individuals in times past to grapple with profound spiritual experiences—namely, the expression of The Divine in mortal lives—and how to tie that back to a comprehensive way of thinking about the world. A template, perhaps, for individuals of our age not to reproduce exactly, but rather to inform our experiences on the path.