Heb 9:14 -
Who through the eternal Spirit - This expression is understood two ways:
1. Of the Holy Ghost himself. As Christ’s miraculous conception was by the Holy Spirit, and he wrought all his miracles by the Spirit of God, so his death or final offering was made through or by the eternal Spirit; and by that Spirit he was raised from the dead, 1Pe_3:18. Indeed, through the whole of his life be was justified by the Spirit; and we find that in this great work of human redemption, the Father, the Son, and the Holy Spirit were continually employed: therefore the words may be understood of the Holy Spirit properly.
2. Of the eternal Logos or Deity which dwelt in the man Christ Jesus, through the energy of which the offering of his humanity became an infinitely meritorious victim; therefore the Deity of Christ is here intended.
But we cannot well consider one of these distinct from the other; and hence probably arose the various readings in the MSS. and versions on this article. Instead of δια Πνευματος αιωνιου, by the Eternal Spirit, δια Πνευματος Ἁγιου, by the Holy Spirit, is the reading of D*, and more than twenty others of good note, besides the Coptic, Slavonic, Vulgate, two copies of the Itala, Cyril, Athanasius sometimes, Damascenus, Chrysostom, and some others. But the common reading is supported by ABD**, and others, besides the Syriac, all the Arabic, Armenian, Ethiopic, Athanasius generally, Theodoret, Theophylact, and Ambrosius. This, therefore, is the reading that should he preferred, as it is probable that the Holy Ghost, not the Logos, is what the apostle had more immediately in view. But still we must say, that the Holy Spirit, with the eternal Logos, and the almighty Father, equally concurred in offering up the sacrifice of the human nature of Christ, in order to make atonement for the sin of the world.

(1) That which regards it as referring to the Holy Spirit, the third person of the Trinity. This was the opinion of Owen, Doddridge, and archbishop Tillotson.
(2) That which refers it to the “divine nature” of Christ. Among those who have maintained this opinion, are Beza, Ernesti, Wolf, Vitringa, Storr, and the late Dr. John P. Wilson. mss. Notes.
(3) Others, as Grotius, Rosenmuller, Koppe, understand it as meaning “endless” or “immortal life,” in contradistinction from the Jewish sacrifices which were of a perishable nature, and which needed so often to be repeated.
(4) Others regard it as referring to the glorified person of the Saviour, meaning that in his exalted, or spiritual station in heaven, he presents the efficacy of his blood.
(5) Others suppose that it means “divine influence,” and that the idea is, that Christ was actuated and filled with a divine influence when he offered up himself as a sacrifice; an influence which was not of a temporal and fleeting nature, but which was eternal in its efficacy. This is the interpretation preferred by Prof. Stuart.

Through the eternal Spirit (dia pneumatos aiōniou). Not the Holy Spirit, but Christ’s own spirit which is eternal as he is. There is thus a moral quality in the blood of Christ not in that of other sacrifices.

Through the eternal spirit (διὰ πνεύματος αἰωνίου)
For the rend. an. Διὰ through = by virtue of. Not the Holy Spirit, who is never so designated, but Christ's own human spirit.

G4151
πνεῦμα
pneuma
pnyoo'-mah
From G4154; a current of air, that is, breath (blast) or a breeze; by analogy or figuratively a spirit, that is, (human) the rational soul, (by implication) vital principle, mental disposition, etc., or (superhuman) an angel, daemon, or (divine) God, Christ’s spirit, the Holy spirit: -ghost, life, spirit (-ual, -ually), mind.
(Strong's)