For by one offering - By offering himself once on the cross. The Jewish priest offered his sacrifices often, and still they did not avail to put away sin; the Saviour made one sacrifice, and it was sufficient for the sins of the world.
He hath perfected forever - He hath laid the foundation of the eternal perfection. The offering is of such a character that it secures their final freedom from sin, and will make them forever holy. It cannot mean that those for whom he died are made at once perfectly holy, for that is not true; but the idea is, that the offering was complete, and did not need to be repeated; and that it was of such a nature as entirely to remove the penalty due to sin, and to lay the foundation for their final and eternal holiness. The offerings made under the Jewish Law were so defective that there was a necessity for repeating them every day; the offering made by the Saviour was so perfect that it needed not to be repeated, and that it secured the complete and final salvation of those who availed themselves of it.
Them that are sanctified - Those who are made holy by that offering. It does not mean that they are as yet "wholly" sanctified, but that they have been brought under the influence of that gospel which sanctifies and saves; see Hebrews 2:11; Hebrews 9:14. The doctrine taught in this verse is, that all those who are in any measure sanctified will be perfected forever. It is not a temporary work which has been begun in their souls, but one which is designed to be carried forward to perfection. In the atonement made by the Redeemer there is the foundation laid for their eternal perfection, and it was with reference to that, that it was offered. Respecting this work and the consequences of it, we may remark, that there is:
(1) perfection in its nature, it being of such a character that it needs not to be repeated;
(2) there is perfection in regard to the pardon of sin - all past sins being forgiven to those who embrace it, and being forever forgiven; and
(3) there is to be absolute perfection for them forever.
They will be made perfect at some future period, and when that shall take place it will be to continue forever and ever.
(The perfection, in this place, is not to be understood of the perfection of grace or of glory. It is perfection, in regard to the matter in hand, in regard to what was the chief design of sacrifices, namely, expiation and consequent pardon and acceptance of God. And this indeed is the Τελειωσις Teleiōsis of the Epistle to the Hebrews generally, Hebrews 7:11; Hebrews 9:9; Hebrews 10:1. Perfect moral purity and consummate happiness will doubtless follow as consequences of the sacrifice of Christ, but the completeness of his expiation, and its power to bring pardon and peace to the guilty and trembling sinner, to justify him unto eternal life, is here, at all events, principally intended. The parties thus perfected or completely justified, are τους ἁγιαζομενους tous hagiazomenous, the "sanctified." Ἁγιαζω Hagiazō, however, besides the general sense of "sanctify" has in this Epistle, like τελειοω teleioō, its sacrificial sense of cleansing from guilt. "Whether ceremonially, as under the Levitical dispensation; Hebrews 9:13; comp, Leviticus 16:19; or really and truly, by the offering of the body of Christ; Hebrews 10:10, Hebrews 10:14, Hebrews 10:29; compare Hebrews 10:2, and Hebrews 2:11; Hebrews 9:14." - Parkhurst's Greek Lexicon. The meaning, then, may be, that they who are purged or cleansed by this sacrifice, in other words, those to whom its virtue is applied, are perfectly justified.
Wherever this divine remedy is used, it will effectually save. By one offering Christ hath forever justified such as are purged or cleansed by it. This could not be said of those sanctified or purged by the legal sacrifices. Mr. Scott gives the sacrificial sense of the word, but combines with it the sense of sanctifying morally, in the following excellent paraphrase. "By his one oblation he hath provided effectually for the perfect justification unto eternal life, of all those who should ever receive his atonement, by faith springing from regeneration, and evidenced 'by the sanctification of the Spirit unto obedience,' and who were thus set apart and consecrated to the service of God.")

For by one offering - His death upon the cross.
He hath perfected for ever - He has procured remission of sins and holiness; fur it is well observed here, and in several parts of this epistle, that τελειοω, to make perfect, is the same as αφεσιν ἁμαρτιων ποιεω, to procure remission of sins.
Them that are sanctified - Τους ἁγιαζομενους· Them that have received the sprinkling of the blood of this offering. These, therefore, receiving redemption through that blood, have no need of any other offering; as this was a complete atonement, purification, and title to eternal glory.

For by one offering,.... The same as before; himself, body and soul; this is a reason why he is set down, and will continue so for ever, and why he expects his enemies to be made his footstool; because by one sacrifice for sin, which he has once offered,
he hath perfected for ever them that are sanctified; that is, who are sanctified by God the Father, Jde 1:1 or, who are set apart by him in eternal election, from the rest of the world, for his own use, service, and glory, to a state of grace and holiness here, and happiness hereafter; for this is not to be understood either of their being sanctified in Christ, though the Syriac version reads, "that are sanctified" in him, or by his Spirit, though both are true of the same persons; these Christ, by his sacrifice, has perfected, and has perfectly fulfilled the law for them; he has perfectly expiated their sins; he has obtained the full pardon of all their sins, and complete redemption; he has perfectly justified them from all things, and that for ever; which shows the continued virtue of Christ's sacrifice, in all generations, to all the elect of God, and the fulness and duration of their salvation; and so Christ by his one sacrifice did what the law, and all its sacrifices, could not do, Hebrews 10:1.

For--The sacrifice being "for ever" in its efficacy (Hebrews 10:12) needs no renewal.
them that are sanctified--rather as Greek, "them that are being sanctified." The sanctification (consecration to God) of the elect (1-Peter 1:2) believers is perfect in Christ once for all (see on Hebrews 10:10). (Contrast the law, Hebrews 7:19; Hebrews 9:9; Hebrews 10:1). The development of that sanctification is progressive.

He hath perfected them for ever - That is, has done all that was needful in order to their full reconciliation with God.