'According to the Spirit of Holiness. The word hagios, holy, when God is spoken of, not only denotes the blameless rectitude in action, but the very Godhead, or to speak more properly, the divinity, or excellence of the Divine nature. Hence hagiosune (the word here used) has a kind of middle sense between hagiotes, holiness, and hagiasmos, sanctification. Comp. Heb. xii.10 (hagiotes or holiness), v.14 (hagiasmos or sanctification). So that there are, as it were, three degrees: sanctification, sanctity of life, holiness. Holiness is ascribed to the Father, the Son, and the Holy Ghost. And since here the Holy Spirit is not mentioned, but the spirit of holiness (prop. sanctity, hagiosune), we must further inquire what this remarkable expression denotes. The name spirit is expressly and very frequently given to the Holy Spirit; but God is also called a spirit; and the Lord Jesus Christ is called a spirit, but in contrast to the latter. (2 Cor. iii.17.) With this we must compare the fact that, as in this passage, so often the antithesis of flesh and spirit is found where Christ is spoken of. (1 Tim. iii.16; 1 Pet. iii.18.) In these passages the Spirit is applied to whatever belongs to Christ (apart from the flesh, although this was pure and holy, and above the flesh), through His generation of the Father, who sanctified Him: in short, His Godhead itself. For here, flesh and spirit, and chap. ix.5, flesh and Godhead, stand in mutual contrast. This spirit is here called not the spirit of holiness, the usual title of the Holy Spirit; but it is called in this passage the spirit of sanctity, to suggest at once the efficacy of that holiness or divinity, which led of necessity to the Saviour's resurrection, and by which it was most forcibly illustrated, and also that spiritual and holy, or Divine power of Jesus who has been glorified and yet retained a spiritual body. Before the resurrection the spirit was concealed under the flesh; after the resurrection the spirit of sanctity concealed the flesh. In reference to the former, He was wont to call Himself the Son of man; in reference to the latter, He is known as the Son of God.'

Beck, in his Lehrwissenschaft, p.604, puts it very clearly, thus --

'Inasmuch as the innocence and purity of Christ were not present in His sufferings and death as a quiescent attribute, but were in full action in the indestructible life-power of the Spirit, as He sanctified His own self to God for us ("through the eternal spirit," Heb. ix.14 -- therefore, in Rom. i.4, hagiosune, the habit of holiness in its action or sanctity, not hagiotes, only an inner attribute, or hagiasmos, holiness in its formation) -- His suffering effected an everlasting redemption.'