22. And the key of the house of David. This expression is metaphorical, and we need not spend much time, as some do, in drawing from it an allegorical meaning; for it is taken from an ordinary custom of men. The keys of the house are delivered to those who are appointed to be stewards, that they may have the full power of opening and shutting according to their own pleasure. By “the house of David” is meant “the royal house.” This mode of expression was customary among the people, because it had been promised to David that his kingdom would be for ever. (2 Samuel 7:13; Psalm 132:11, 12.) That is the reason why the kingdom was commonly called “the house of David.”
The key is put in the singular number for keys. Though “keys” are usually carried in the hands, yet he says that they are laid on the shoulders, because he is describing an important charge. Yet nothing more is meant than that the charge and the whole government of the house are committed to him, that he may regulate everything according to his pleasure; and we know that the delivering of keys is commonly regarded as a token of possession.
Some commentators have viewed this passage as referring to Christ, but improperly; for the Prophet draws a comparison between two men, Shebna and Eliakim. Shebna shall be deprived of his office, and Eliakim shall succeed him. What has this to do with Christ? For Eliakim was not a type of Christ, and the Prophet does not here describe any hidden mystery, but borrows a comparison from the ordinary practice of men, as if the keys were delivered to one who has been appointed to be steward, as has been already said. For the same reason Christ calls the office of teaching the word, (Matt. 16:19,) “the keys of the kingdom of heaven;” so that it is idle and foolish to spend much time in endeavouring to find a hidden reason, when the matter is plain, and needs no ingenuity. The reason is, that ministers, by the preaching of the word, open the entrance into heaven, and lead to Christ, who alone is “the way.” (John 14:6.) By the keys, therefore, he means here the government of the king’s house, because the principal charge of it would be delivered to Eliakim at the proper time.
22. key—emblem of his office over the house; to “open” or “shut”; access rested with him.
upon … shoulder—So keys are carried sometimes in the East, hanging from the kerchief on the shoulder. But the phrase is rather figurative for sustaining the government on one’s shoulders. Eliakim, as his name implies, is here plainly a type of the God-man Christ, the son of “David,” of whom Isaiah (Is 9:6) uses the same language as the former clause of this verse. In Rev 3:7, the same language as the latter clause is found (compare Job 12:14).
Ver. 22.—The key of the house of David will I lay upon his shoulder. A key would seem to have been the special badge of the prefect’s office, which included the control of the stores (ver. 15), and the general management of the household. It was, perhaps, a part of the form of investiture, that the key should be first laid on the prefect’s shoulder and then delivered into his hand. Among the Greeks the priests of Ceres are said to have borne a key on their shoulder, permanently, as a badge of office (Callimach., ‘Hymn. ad Cererem,’ l. 45). The reference to this passage in Rev. 3:7 is sufficient to show that Elakim, the “servant of Jehovah” (ver 20), is, to a certain extent, a type of Christ; perhaps also of his faithful ministers (Matt. 16:19; John 20:23).
 Jamieson, R., Fausset, A. R., & Brown, D. (1997). Commentary Critical and Explanatory on the Whole Bible (Vol. 1, p. 455). Oak Harbor, WA: Logos Research Systems, Inc.