4. And declared--literally, "marked off," "defined," "determined," that is, "shown," or "proved."
      to be the Son of God--Observe how studiously the language changes here. He "was MADE [says the apostle] of the seed of David, according to the flesh" (Ro 1:3); but He was not made, He was only "declared [or proved] to BE the Son of God." So Joh 1:1, 14, "In the beginning WAS the Word . . . and the Word was MADE flesh"; and Isa 9:6, "Unto us a Child is BORN, unto us a Son is GIVEN." Thus the Sonship of Christ is in no proper sense a born relationship to the Father, as some, otherwise sound divines, conceive of it. By His birth in the flesh, that Sonship, which was essential and uncreated, merely effloresced into palpable manifestation. (See on Lu 1:35; Ac 13:32, 33).
      with power--This may either be connected with "declared," and then the meaning will be "powerfully declared" [LUTHER, BEZA, BENGEL, FRITZSCHE, ALFORD, &c.]; or (as in our version, and as we think rightly) with "the Son of God," and then the sense is, "declared to be the Son of God" in possession of that "power" which belonged to Him as the only-begotten of the Father, no longer shrouded as in the days of His flesh, but "by His resurrection from the dead" gloriously displayed and henceforth to be for ever exerted in this nature of ours [Vulgate, CALVIN, HODGE, PHILIPPI, MEHRING, &c.].
      according to the spirit of holiness--If "according to the flesh" means here, "in His human nature," this uncommon expression must mean "in His other nature," which we have seen to be that "of the Son of God"--an eternal, uncreated nature. This is here styled the "spirit," as an impalpable and immaterial nature (Joh 4:24), and "the spirit of holiness," probably in absolute contrast with that "likeness, of sinful flesh" which He assumed. One is apt to wonder that if this be the meaning, it was not expressed more simply. But if the apostle had said "He was declared to be the Son of God according to the Holy Spirit," the reader would have thought he meant "the Holy Ghost"; and it seems to have been just to avoid this misapprehension that he used the rare expression, "the spirit of holiness."

JFB.