The "perfected forever" more refers to thefact that the sacrifice of Jesus on the cross was once for all time. No longer needs to be offered annually as was the case under the old covenant. The blood of Jesus cleanses the new believer, the repentant sinner who comes back to the Lord, and the mature believer, all believers.
Lets look at the "perfected forever" and the context in which it is found.
Hebrews 10 11 And every priest stands ministering daily and offering
repeatedly the same sacrifices,
which can never take away sins. 12 But this Man, after He had offered
one sacrifice for sins forever, sat down at the right hand of God, 13 from that time waiting till His enemies are made His footstool. 14
For by one offering He has perfected forever those who are being [
d]sanctified.
It refers to the fact that Jesus sacrifice is PERFECT, and has no need to be repeated annually as per the old covenant with the high priest. This sacrifice covers the new believer, the old believer, the backslidden but now repentant believer. Jesus no longer needs to go to the cross again and again.
Now lets look a little further in Hebrews 10:
19 Therefore, brethren, having boldness[
f] to enter the Holiest by the blood of Jesus, 20 by a new and living way which He consecrated for us, through the veil, that is, His flesh, 21 and
having a High Priest over the house of God, 22 let us draw near with a true heart in full assurance of faith, having our hearts sprinkled from an evil conscience and our bodies washed with pure water. 23
Let us hold fast the confession of our hope without wavering, for He who promised
is faithful. 24 And let us consider one another in order to stir up love and good works, 25 not forsaking the assembling of ourselves together, as
is the manner of some, but exhorting
one another, and so much the more as you see the Day approaching.
The Just Live by Faith
26 For if we sin willfully after we have received the knowledge of the truth, there no longer remains a sacrifice for sins, 27 but a certain fearful expectation of judgment, and fiery indignation which will devour the adversaries. 28 Anyone who has rejected Moses’ law dies without mercy on
the testimony of two or three witnesses. 29
Of how much worse punishment, do you suppose, will he be thought worthy who has trampled the Son of God underfoot, counted the blood of the covenant by which he was sanctified a common thing, and insulted the Spirit of grace? 30 For we know Him who said, “Vengeance is Mine, I will repay,” [g]says the Lord. And again, “The Lord will judge His people.” 31 It is a fearful thing to fall into the hands of the living God.