Prophet heightens the description of that which he had already declared
COMMENTARY ON ISAIAH
the Gospel may be called a "chariot," because it conveys us to the hope of eternal life; but I think that the Prophet simply declares that nothing shall hinder God from gathering his Church, and that he will have at his command all the necessary means, that none of the elect whom he has called may fail in the middle of the course.
21. And I will even take some of them for priests and Levites. The Prophet heightens the description of that which he had already declared about the extraordinary grace of God. He had made known that the Church of God should be collected out of all nations, so that, in spite of every difficulty and obstruction, even distant nations should draw near to them. But now he proceeds further, and instructs them that the Gentiles shall not only be adopted by God, but shall also be elevated by him to the highest honor. Already it was a great honor, that unclean and polluted nations were reckoned to be a holy people; but now here is something far more wonderful, that they are elevated to the highest pinnacle of rank.
Hence we see that the priesthood under Christ is very different from what it was under the Law; for under the Law one tribe exclusively was admitted to the priesthood, and the Gentiles, as unclean, were so far from having it in their power to discharge that priesthood, that they were even forbidden to enter into the temple; but now all are admitted without distinction. Some expound this passage in a general manner, that the Gentiles shall be priests; that is, shall offer themselves to God, as Scripture frequently denominates all believers "a royal priesthood." (1 Peter 2:9; Revelation 1:6; 5:10.) But he appears to describe in an especial manner ministers and teachers whom the Lord also chose from among the Gentiles, and appointed to execute this distinguished office; that is, to preach the Gospel; such as Luke, Timothy, and others of the same class, who offered spiritual sacrifices to God by the Gospel.
22. For as the new heavens. Here he promises that the restoration of the Church shall be of such a nature that it shall last for ever. Many might be afraid that it would be ruined a second time; and therefore he declares that henceforth, after having been restored by God, its condition shall be permanent. Accordingly, he mentions here two benefits of surpassing excellence, restoration and eternity. When he speaks of "new heavens" and a "new earth," he looks to the reign of Christ, by whom all things have been renewed, as the Apostle teaches in the Epistle to the Hebrews. Now the design of this newness is, that the condition of the Church may always continue to be prosperous and happy. What is old tends to decay; what is restored and renewed must be of longer continuance. (Hebrews 8:13.)
So shall your seed and your name remain. God had promised that "the sun and moon," so long as they remained in heaven, should be witnesses of the eternal succession, that the posterity of David might not be cut off. But because some interruption arose from the treachery and ingratitude of the people, the restoration effected by Christ actually confirmed that prediction.