‘It is interesting that these verses all say that believers “are sanctified” in the perfect tense (Acts 20:32; Acts 26:18; 1 Cor. 1:2; Heb. 2:11; Heb. 10:10; Jude 1:1).
DEFINITION OF PERFECT TENSE: “The perfect tense in Greek corresponds to the perfect tense in English, and describes an action which is viewed as having been completed in the past, once and for all, not needing to be repeated. Jesus’ last cry from the cross, TETELESTAI (“It is finished!”) is a good example of the perfect tense used in this sense, namely “It [the atonement] has been accomplished, completely, once and for all time.”” (Blueletterbible.com)
The word in these passages is “ἁγιάζω” and it means to be consecrated or dedicated to God. The use of the perfect tense means that believers have been completely sanctified or that their sanctification has been accomplished completely.
If sanctification were a present or ongoing process the New Testament would not have used the perfect tense in these verses or at least would have used the imperfect tense to express that a durative process had taken place.
These verses do not teach that believers “are slowly being sanctified” in a on-going life-long process, but that they have been definitely sanctified completely in the past. It speaks of the believers sanctification as something that has come to completion hence the use of the Greek perfect tense.
What exactly would a progressive sanctification look like? This year you repent of fornication and then maybe next year you repent of lying? And then the next year you repent of another sin? As I understand it, when you truly repent you repent of all the sins you are involved in. Now, if a believer returns to their sins they are for the time being impenitent and need to repent and be justified and sanctified again. But you cannot be justified and sanctified while you are impenitent or in sin.’
Source: Jesse Morrell, “Entire Sanctification or Christian Perfection” (biblicaltruthresources)
Read also “Is Sanctification A Process?” by Jamie RA Gerrard.