Context shows us that the person sanctified by the blood of Christ is indeed a saved person:
"10 By this will we have been sanctified through the offering of the body of Jesus Christ once for all." - Hebrews 10:10
"14 For by one offering He has perfected for all time those who are sanctified." - Hebrews 10:14
"29 How much severer punishment do you think he will deserve who has trampled under foot the Son of God, and has regarded as unclean the blood of the covenant by which he was sanctified, and has insulted the Spirit of grace?" - Hebrews 10:29
This also shows us that being made perfect forever in Christ doesn't mean you can't lose the perfection He gives, but actually means what the chapter itself says it means, that Christ doesn't have to be repeatedly sacrificed. The Sacrifice lasts forever and doesn't need to be repeated. That's how it is 'forever'.
It doesn't say you possess it forever. It says nothing about forever possessing the Sacrifice that lasts forever, no matter what:
"1 For the Law, since it has only a shadow of the good things to come and not the very form of things, can never, by the same sacrifices which they offer continually year by year, make perfect those who draw near. 2 Otherwise, would they not have ceased to be offered, because the worshipers, having once been cleansed, would no longer have had consciousness of sins?" - Hebrews 10:1-2
You only possess the perfection of Christ's one time for all time Sacrifice forever
if you continue to believe and trust in it.
That's why the author tells us to continue to believe in the Sacrifice and ministry that lasts forever and does not need to be repeated:
"23 Let us hold fast the confession of our hope without wavering, for He who promised is faithful" - Hebrews 10:23
"14 Therefore, since we have a great high priest who has passed through the heavens, Jesus the Son of God, let us hold fast our confession." - Hebrews 4:14
Because Christ's Sacrifice is so great and makes us perfect in God's sight and lasts forever, not needing to be repeated like under the old system, we should hold fast the confession of such a great High Priest and ministry. The chapter says nothing about 'forever' meaning you can never lose the effect of Christ's ministry no matter what.