Homoousios (Greek: ‘of the same substance/being’) is the key term of the Nicene Creed — the Son is ‘of the same substance as the Father.’ This single term, inserted into the creed by Emperor Constantine on the advice of Hosius of Cordoba, became the defining line between orthodoxy and heresy for all subsequent Western and Eastern Christianity. Its problem: the word homoousios does not appear in Scripture. It was a philosophical term, which made it suspect to many biblicists. It was also the term used (and condemned) by Paul of Samosata in a Modalistic context a generation earlier, which made its adoption deeply suspicious to many Eastern bishops. Athanasius of Alexandria was its most tenacious defender — arguing that only if the Son is truly God can the incarnation actually save humanity (the ‘deification’ logic: what is not assumed is not healed).