Hebrews 9:14

How much more shall the blood of Christ
Which is not the blood of a mere man, but the blood of the Son of God; and the argument is from the lesser to the greater; that if the ashes of the burnt heifer, which was a type of Christ in his sufferings, mixed with water, typically sanctified to the purifying of men externally, in a ceremonial way, then much more virtue must there be in the blood of Christ, to cleanse the soul inwardly:

who through the eternal Spirit offered himself without spot to God;
Christ is a priest, and the sacrifice he has offend up is "himself"; not his divine nature, but his human nature, soul and body, as in union with his divine person; which gives his sacrifice the preference to all others; and is the reason of its virtue and efficacy, and is expressive of his great love to man: and this sacrifice was offered up "to God", against whom his people had sinned, and whose justice must be satisfied, and which is of a sweet smelling savour to him; besides, he called him to this work, and engaged him in it, and is well pleased with this offering, as he must needs be, since it is offered up "without spot"; which expresses the purity of Christ's nature and sacrifice, and the perfection of it, which is such, that no fault can be found in it by the justice of God; and hence, the saints, for whom it is offered, are unblamable and irreprovable, There is an allusion in the clause, both to the priests and to their sacrifices, which were neither of them to have any spot or blemish on them; and this unblemished sacrifice was offered unto God by Christ,

through the eternal Spirit;
not the human soul of Christ; for though that is a spirit, yet not eternal, and besides, was a part of the sacrifice; but rather the divine nature of Christ, which is a spirit, and may be so called in distinction from the flesh, or human nature, as it sometimes is, and this is eternal; it was from everlasting, as well as is to everlasting; and this supported him under all his sufferings, and carried him through them, and put virtue unto them; and Christ was a priest, in the divine, as well as human nature: though by it may be better understood "the Holy Ghost"; and so the Vulgate Latin version reads, and also several copies; since the divine nature rather acts by the human nature, than the human nature by the divine; and Christ is often said to do such and such things by the Holy Spirit; and as the Holy Ghost formed and filled the human nature of Christ, so he assisted and supported it under sufferings. This whole clause is inserted by way of parenthesis, showing the efficacy of Christ's blood, and from whence it is:

to purge your conscience from dead works;
that is, "from the works of sin", as the Ethiopic version renders it; which are performed by dead men, separate and alienated from the life of God, are the cause of the death of the soul, and expose to eternal death, and are like dead carcasses, nauseous and infectious; and even duties themselves, performed without faith and love, are dead works; nor can they procure life, and being depended on, issue in death; and even the works of believers themselves are sometimes performed in a very lifeless manner, and are attended with sin and pollution, and need purging: the allusion is to the pollution by the touch of dead bodies; and there may be some respect to the sacrifices of slain beasts, after the sacrifice and death of Christ, by believing Jews, who were sticklers for the ceremonies of the law, and thereby contracted guilt; but immoralities are chiefly designed, and with these the conscience of man is defiled; and nothing short of the blood of Christ can remove the pollution of sin; as that being shed procures atonement, and so purges away the guilt of sin, or makes reconciliation for it, so being sprinkled on the conscience by the Spirit of God, it speaks peace and pardon, and pacifies and purges it, and removes every incumbrance from it: the Alexandrian copy, the Vulgate Latin, and Syriac versions, read, "our conscience". The end and use of such purgation is, "to serve the living God"; so called to distinguish him from the idols of the Gentiles, and in opposition to dead works; and because he has life in himself, essentially and independently, and is the author and giver of life to others; and it is but the reasonable service of his people, to present their souls and bodies as a living sacrifice to him; and who ought to serve him in a lively manner, in faith, and with fervency, and not with a slavish, but a godly filial fear; and one that has his conscience purged by the blood of Christ, and is sensibly impressed with a discovery of pardoning grace, is in the best capacity for such service. The Alexandrian copy reads, "the living and true God".