Sentence:  They therefore devoted themselves to examining the nature of the soul, and taught that its freedom consists in communion with God, to be achieved by absorption in a sort of ecstatic trance. This doctrine reaches its height in Plotinus, after whom it degenerated into magic and <WSD>theurgy</WSD> in its unsuccessful combat with the victorious Christianity. 
Meaning of the word marked with <WSD> tags:  A form of magic designed to allow for worship or conjuration of, or communication with spirits or deities.