In heb 10:
10By the which will we are sanctified through the offering of the body of Jesus Christ once for all.
Here, sanctified is in the perfect tense, it has be done once and for all in the past with results into the present.
In heb 10:14 The word Perfected, is in the perfect tense as well, and so, by these verbs being perfect, that would cause a continual sanctification. of body, soul, and Spirit until that Perfect day, the day of Christ.
1 thess 5:
23And the very God of peace sanctify you wholly; and I pray God your whole spirit and soul and body be preserved blameless unto the coming of our Lord Jesus Christ.
24Faithful is he that calleth you, who also will do it.
You are correct! The verb used in verse 10 is indeed in the perfect tense. Which is to say, that what Christ did is done. Christ sanctified those who believe. But in context we see that those who believe are being sanctified.
Huh? Is that double talk? No. It is just the way God works. He knows those who are going to believe. He has seen the end before the beginning. Therefore, we can have boldness to come before the throne and receive this faith that He offers. Knowing that He who has started this, is more than able to finish it.
But, unfortunately, verse 14 is not in the perfect tense. It is in the present tense. Which is to say it is "continuous". This is where we see the context.
Hebrews 10:14 Greek Study Bible (Apostolic / Interlinear)
μιᾷ γὰρ προσφορᾷ τετελείωκεν εἰς τὸ διηνεκὲς τοὺς ἁγιαζομένους.
KJV with Strong's
For by one offering he hath perfected for ever them that are sanctified
Greek Transliteration Strong's Morphology English
μιᾷ mia 1520 A-DSF
one
γὰρ gar 1063 CONJ
For
προσφορᾷ prosphora 4376 N-DSF
offering
τετελείωκεν teteleiōken 5048 V-RAI-3S
he has fully perfected
εἰς eis 1519 PREP
for
τὸ to 3588 T-ASN
those
διηνεκὲς diēnekes 1336 A-ASN
ever
τοὺς tous 3588 T-APM
those
ἁγιαζομένους agiazomenous 37
V-PPP-APM being sanctified
I want to draw your attention to the bold and underlined letters. They represent (V)Verb-(PPP)Present Passive Participle-(APM)Accusative Plural Masculine
But what we do see is that the full, 'perfect tense' of the sanctification in verse 10, is applied to those here in verse 14 who are being sanctified. That is, the sacrifice has been made, it just needs to be continually sprinkled onto the heart of the believer. But we know from the morphology of the original language that this is an ongoing process, the sprinkling, not the sacrifice being made.