Jesus Christ, with His one offering, perfected all those who are being sanctified, i.e., His people (Heb.10:14). Christ’s one offering of Himself stands in contrast to the countless number of sacrifices made under the law. Those sacrifices, though made repeatedly, could never take away sins (10:11), and make those who offered them perfect (10:1). The writer of Hebrews uses the term “perfect” not in the sense of being perfect as in sinless, but in the sense of being perfectly or completely forgiven of sins. Under the Old Covenant one could not achieve that kind of perfection (7:11, 19, 10:1). It was under the New Covenant that God made perfect His people through the sacrifice of Jesus Christ. Therefore, even His people who committed sins under the first covenant were not made perfect or completely forgiven until Christ came and died for their sins (9:15, 11:40, 12:23). God promised under the New Covenant that He would remember the sins and lawless deeds of His people no more. And because Christ completely atoned for their sins, there is no longer an offering for sin (Heb.10:14-18).
The writer of “Hebrews” is making an argument to these first-century Jewish people who had professed faith in Christ, but were in danger of drifting away from him back to Judaism. Since Christ came and atoned for all the sins of His people, there is no longer a need to return to offering animals which are no longer legitimate sacrifices for sin. To do so would be to trample underfoot the Son of God, to count HIs blood a common thing, and to be worthy of God’s judgment (10:29).