Monopatrism is the belief that what became a man was not God's essence but his prime manifestation.
"The Son ... cannot serve as an intermediary between the Father and the Spirit."
Photios 1 of Constantinople, Encyclical, 867 CE.
Filioquism is the belief that it was God's essence that became a man.
"The Holy Spirit ... proceeds from the Father and from the Son."
Benedict 8 of Rome, Supplemented Creed of Constantinople, 1014.
Monopatrism is the most subtle of the historic forms of Atheanthroposialism; indeed it is hard to imagine a more subtle. It pushes its denial back "through all ranks of creatures, to the central height". It claims to affirm that Christ is God, but then, in effect, adds "well, near enough".
Here are some relevant dates.
381 The Creed of Constantinople, a modified version of the 325 Creed of Nicea, mentioned that "the holy spirit proceeds from the father". This creed was quickly adopted by most Christianoids.
589 (or thereabouts) Christianoids began to supplement "from the father" (a patre) with "and from the son" (filioque). Opinions differed on that exact formulation, but there was consensus that the Son had some role in the Spirit's generation, if not on what his role was, and the issue was generally not divisive. (589 is the year of the Canons of Toledo; its original text may not have included the filioque, in which case the relevant date may have been somewhat later.)
867 Photius declared that the father alone generated the spirit, that the son had no role.
1014 Benedict 8 of Rome authorised the inclusion of the filioque in a ceremony regarded by all concerned at the time as central to Christianoid practice. (To understand the historic significance of this we must remember that they were all Magicalists. Also a metaheresy of course, but for present purposes that is irrelevant.) I have taken this as the key reformational event, though others could equally have been chosen.
1054 This issue helped crystallise conflict between Western Christianoids ("Western Catholics", led from Rome, which was Filioquist) and their Eastern counterparts ("Eastern Orthodox", led from Constantinopole, which was Monopatrist) into the East-West schism, which remains open.
1330 (or thereabouts) Monopatrism was consolidated by Gregory Palamas in his theory of the divine manifestations as "energies".
1351 A Council in Constantinople adopted Palamas' theory so it became official "Eastern Orthodox" doctrine.
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