- Basically, Granville Sharp's rule states that when you have two nouns, which are not proper names (such as Cephas, or Paul, or Timothy), which are describing a person, and the two nouns are connected by the word "and," and the first noun has the article ("the") while the second does not, both nouns are referring to the same person.
In our texts, this is demonstrated by the words "God" and "Savior" at Titus 2:13 and 2 Peter 1:1. "God" has the article, it is followed by the word for "and," and the word "Savior" does not have the article. Hence, both nouns are being applied to the same person, Jesus Christ.
White, James, Granville Sharp's Rule: Titus 2:13, 2 Peter 1:1 (online article.)
However, the Granville Sharp Rule is by no means the be-all and end-all of Greek grammar. It is not immutable, as many Trinitarians appear to believe.
Indeed, the only publication which Sharp wrote on any aspect of NT grammar was a monograph on the Greek article, appearing in 1798 and bearing the title Remarks on the Uses of the Definitive Article in the Greek Text of the New Testament: Containing many New Proofs of the Divinity of Christ, from Passages which are wrongly Translated in the Common English Version.
It consisted of less than 60 pages.
This little piece of writing (published more than two centuries ago) contained Sharp’s famous Rule, which he believed to be consistent without exception. That was certainly true of the New Testament – but even in Sharp’s own lifetime, another grammarian (Calvin Winstanley) was able to produce four classes of exceptions to Sharp’s rule in Greek literature outside the NT.
His exceptions have been largely ignored because they do not refer to passages of Scripture – but this is irrelevant, because the argument is predicated on a principle of grammar and not the material in which it is found.
The bottom line is that exceptions to Sharp’s rule were found and for this reason, it cannot be considered irrefutable.