Theurgia, theos (god) plus ergon (work), was coined in the Chaldean Oracles milieu and systematized by Iamblichus (c. 245-325 CE), who argued prayer and rite do not persuade gods but align the soul with them. It became the Neoplatonic sacramental practice and the ancestor of later ceremonial magic.
The working distinction every magician quotes: thaumaturgy works wonders, theurgy works the worker. The high/low split lets practitioners rebrand the same candle as philosophy.
Ergon, work, built energy (at-work), organ (the working part), ergonomics, and surgeon (hand-worker, kheir-ergon). Theurgy is the spiritual member of the work family: the claim that ritual is a kind of labor, and the soul its workplace.
Is ritual a technology for changing the world, or for changing the operator?