E.C. Colwell, a Trinitarian Methodist, was apparently affected by various translators of his time and before, that were translating John 1:1 differently from the norm: "the Word was God." You had Moffatt’s version saying "the Word was divine" (1922); a French Bible, "a divine being" (1928); Belsham, "a god" (1808); Benjamin Wilson: "a god" (1864); Robert Young: "a god" in his Commentary of 1985. Colwell likely knew that various French and German Bibles were being published challenging the traditional rendering "God" of John 1:1 as well.
So he set out to prove them wrong. He began by formulating a "rule" regarding predicate nouns and the Greek article, whether they occurred after or before the verb. He was looking for Scriptures he understood were "definite" in the English translation, but were lacking the article in Greek when they occurred before the verb, but had it after the verb. In all, he examined 367 predicate nouns. He was influenced by Scriptures such as Matthew 12:48,50 (where one noun, "mother," had the article after the verb and the other before the verb did not), John 1:49 and John 9:5, where this last one didn’t have the article but in John 8:12 did, and he concluded that they were definite all the same. Bingo, someone finally had the weapon to destroy those "heretics," so he thought.
He concluded, "Kai theós en ho lógos" looks much more like "And the Word was God" when viewed with reference to this rule." This was only a "theory" as he labeled it. However, Trinitarians the world over rejoiced in having the ultimate weapon to neutralize the opposition. Walter Martin, Robert H. Countess, and Bruce Metzger, to name three scholars, were ecstatic, and used it repeatedly against the JWs to "prove" their error.
The problem was that the rule was flawed from the beginning, and it didn’t prove these predicate nouns were definite. After the NWT (which ignored the rule) was published, other translators too disregarded Colwell’s Rule, and went on to translate John 1:1, as "divine," "divine being," "god" (instead of God), etc. Not only that, some scholars of repute, like C.H. Dodd, have admitted publicly, that grammar-wise, you could translate both ways.
Grammarians too have pointed out the error of Colwell’s Rule, such as Turner and Wallace. Nonetheless, there are a few remaining scholars who won’t let the "rule" die in peace, like David A. Black, and Köstenberger, Merkle, and Plummer in their book, Going Deeper with NT Greek, who still quote Colwell like it were the Bible. Not so fast!
The problem with Colwell’s Rule is that he went in looking for definite predicate nouns, when from the start it was not clear they were definite to begin with, like in John 1:1. In other words, he used his own criteria or interpretation to determine they were definite. And that ladies and gentlemen is the crux of the problem. What is a definite predicate noun for him, may be qualitative or indefinite to another.
Worse, he did not consider in the main qualitative and indefinite nouns in his study. He also ignored in his paper prepositional phrases, relative clauses, proper nouns, etc. Colwell admitted his "rule" had 15 exceptions in the New Testament. What kind of rule is that, with 15 exceptions? That’s not counting all the ones he considered "definite," which potentially may not be.
In all, Colwell’s Rule should be thrown in the trash heap where it belongs.