Homoousios and Homoiousios

Main articles: Homoousian and Chalcedonian

The First Council of Nicaea in 325 debated the terms homoousios and homoiousios. The word homoousios means "same substance", whereas the word homoiousios means "similar substance". The council affirmed the Father, Son, and Holy Spirit (Godhead) are of the homoousious (same substance). This is the source of the English idiom "differ not by one iota." Note that the words homoousios and homoiousios differ only by one 'i' (or the Greek letter iota). Thus, to say two things differ not one iota, is to say that they are the same substance.