We quote Professor Moule: “In John 20:28 Ho kýrios mou kai ho theós mou [that is, My Lord and my God], it is to be noted that a substantive [like God] in the Nominative case used in a vocative sense [in address to Jesus] and followed by a possessive [of me] could not be anarthrous [that is, without the definite article the] . . . ; the article [the] before theós may, therefore, not be significant. . . . the use of the article [the] with a virtual Vocative (compare John 20:28 referred to above, and 1 Peter 2:18, Colossians 3:18ff.) may also be due to Semitic idiom.”—Pages 116, 117, of An Idiom-Book of New Testament Greek, by C. F. D. Moule, Professor of Divinity in the University of Cambridge, 1953 edition, England.
For instance, to show that a vocative in Greek ordinarily has the definite article before it, we note that in 1 Peter 2:18; 3:1, 7 the literal word-for-word translation reads: “The house servants, be subject . . . In like manner, [the] wives, be . . . The husbands, continue dwelling.” in Colossians 3:18 to 4:1: “The wives . . . The husbands, . . . The children . . . The fathers . . . The slaves . . . The masters.”