Dan Wallace on John 1:1: the articular construction fights Sabellianism; the anarthrous predicate fights Arianism

Wallace argues the emphatic word order of John 1:1c (predicate nominative without article placed before the verb) stresses the essence or quality of the Word — "what God was, the Word was." The definite article on theon in 1:1b keeps us from identifying the Word with the Father (thus fighting Sabellianism/modalism). The absence of the article in 1:1c prevents us from thinking the Word is merely one of many gods or a lesser god (fighting Arianism). Wallace's Greek grammar is the standard text used in seminary Greek programs.