I was reading an article where a Protestant pastor cited Hebrews 10:14 as his primary proof text against the doctrine of Purgatory. The verse says: "For by a single offering he [Jesus] has perfected for all time those who are being sanctified." The pastor's interpretation was something along the lines of: by that one single sacrifice on the Cross, Jesus has perfectly saved us, leaving nothing unfinished, and thus there is no room nor need for us needing forgiveness later on (e.g. such as in Purgatory). This reading is understandable, and quite common for Protestants to make against Catholics. So I think it's a good idea to take a look at how to address this claim.
The first thing I would point out is that Christians can still fall into sin and still need to repent of any new sins (e.g. forgive us our tresspasses), as we see throughout the Bible. The congregations in Corinth and Galatia had fallen into sin and needed to repent (2 Cor 12:21). Jesus even sends John to warn the 'seven churches' of Revelation ch2-ch3 of repenting of their bad behavior. So it is a well-established fact that forgiveness is not something that takes place only once in a Christian's life. Thus, we have good reason to not interpret the "by one offering he perfected" of Heb 10:14 to mean your sins are perfectly forgiven the moment you first accept the Gospel. On top of that, even Protestants admit that our growing in inward holiness is a 'work in progress', since each day we must strive to uproot sin and become more holy, which is a very slow process, meaning Christians are far from perfect. And without the Cross, we would be unable to make any steps towards holiness at all. But then we must admit "the one offering" did not perfect our sanctification, and thus we see a second reason why the Protestant interpretation cannot work against Purgatory. With the Protestant interpretation largely discredited, that opens up the door for us to explore alternative interpretations of what Paul is saying, because it seems like a very big deal to say that the Cross perfects us.
The next reasonable step in our study is to consider the possible meanings of the words that 10:14 uses, because often times we incorrectly assume the modern day English meaning of a Biblical word. The key word of this passage is "perfected," which Greek term is found 24x in the New Testament (here), and has a range of meaning along the lines of "to complete, accomplish, finish, bring to the end goal". If you look at the verses, this Greek term "perfect" is not used in any of these verses to mean nor suggest "without sin, flawless," such that a Christian is absolutely perfect now. Consider that Jesus told his Apostles that He was 'not yet perfect' (Lk 13:32; Heb 5:9; 7:28), which obviously cannot mean Jesus was not yet sinless, but rather that Jesus had yet to attain His final goal (Cross & Resurrection). And Paul says he as a Christian has not attained perfection yet (Phil 3:12), which obviously contradicts Heb 10:14 unless we admit "perfect" can have a range of meaning. So at this point, we can safely say that Heb 10:14 means that Christians have been brought to some goal or accomplishment stage, but that is not a state of sinless perfection.
What is fascinating is that this Greek term "perfect" is actually found in the Greek Old Testament (LXX, short for the Greek Septuagint translation) about 14x (here), and found mostly in Exodus and Leviticus, when talking about the Aaronic/Levitical priesthood. Consider:
The Greek term for "perfect" is translated as "consecrated" in these passages. In fact, it more accuately says says "consecrated his hands," which sounds exactly like how Catholic priests are ordained, they have oil poured on their hands, to be consecrated/enabled to do God's work. The Catholic Douay-Rheims properly translates the Greek and Hebrew as "consecrate his hands" (see here) but few Protestant translations include the word "hands" even though it is in the Greek/Hebrew. Instead, most Protestant translations merely say "consecrate Aaron," leaving out the "hands," or only put it in the footnotes (see here). This is kind of suspicious if you ask me, because while merely using the term "consecrate" is not wrong, hiding the word "hands" almost seems like a way to hide the Catholic priesthood in the Bible.
The Old Testament Hebrew text is more literally saying "fill his hand," as in to fill his hand with power, or to equip his hand, thus, "perfect his hand" is a Hebrew idiom for "ordain" or "consecrate" to now be at the service of liturgical worship in God's temple. This is simply fascinating. It is interesting how there is other priestly functions done by the hands of the angels (Isaiah 6:6-7; Rev 8:3-4).
Now with this in mind, consider anew the context of how the book of Hebrews uses the term "perfect":
Clearly, these passages which mention "made perfect" also use priesthood language like 'appointing', 'offering', 'sacrifices', and the weakness of the Old Testament Levitical Priesthood. This reasonably ties back to the "perfecting of hands," or ordination, of priesthood in the above Exodus/Levitical passages we looked at. Thus, it is most reasonable that when Hebrews says "made perfect," the best interpretation is something along the lines of the Cross and Priesthood of Christ now leads to ordaining, or equipping, or consecrating (set apart), those of the Christian community.
I can see this perfecting ordination/consecration being used narrowly to refer strictly to the Catholic priesthood, or more broadly to everyone playing their own unique role in the Church, since the Church is the Consecrated Body ordained to make the world holy (Eph 5:25-27 says Jesus sprinkled water along with words on His Bride [i.e. baptism and/or ordination]). St Paul tells bishop Timothy that in God's house there are two classes of people, regular folks and 'sacred' folks, who have consecrated themself to holy deeds, which are obviously the ministers (2 Tim 2:20-21), or at minimum not in mortal sin. Thus, we could see in this Hebrews 10 passage an implicit (or shorthand) reference to the institution of the New Testament Catholic Priesthood, as the text of 10:14 would really be saying: 'by Christ's high priesthood, he consecrated/ordained (perfected [the hands]) those who are set apart (sanctified) for Christian worship'.
It is interesting to see how there seems to be some clear parallels in many of the related Hebrews passages, such that we can piece together some more of the meaning. Consider that "perfect the worshiper's conscience" (Heb 9:9) seems to be equivalent to "make perfect those who draw near..the worshiper's conscience" (Heb 10:1-2), and also "perfected those sanctified" (Heb 10:14). Thus, those who "draw near" are feasibly those drawing nearer to the altar, thus are worship leaders, and thus are sanctified. This seems to support the ordained New Testament Catholic Priesthood. Perhaps the message of Hebrews is that the Old Testament Priesthood only effected outward cleansing, while the New Testament Priesthood really does clear the conscience. This is especially true given that the OT Priesthood could not forgive grave sins like murder, adultery, etc, while the New Testament Priesthood can forgive any sin no matter how grave.
But equally, we often forget that the Old Testament vision of "God's People" (the "Elect/Chosen"), was that the individual Israelites saw themselves and their mission as a Community, and did not see salvation or their mission in terms of Individuality. In this way, the "those" in "those who are sanctified" can mean more than the Priesthood, and extend to include the whole Community: i.e. only those sanctified by circumcision were allowed to eat the Passover (Ex 12:48), just as only Christians who have been baptized may eat the Eucharist (this teaching of Baptism before Eucharist comes more from Tradition than the Bible, though even Protestants realize it's true).
To further support this conclusion, consider that the Greek term for "sanctified" appears in about 29x verses in the New Testament (here). The dominant usage for "sanctify" is not so much about growing in internal holiness, but rather an assignment or designation or categorization as a "sacred object" which is set apart for sacred usage in God's service (e.g. 1 Cor 1:2; 6:11; 1 Tim 4:5; Jude 1:1). This is precisely what the designation of "saints" is in the Bible, those who are set apart, rather than those who are inwardly immaculate and sinless. One thing that is ironic about this realization is that the Protestant view of declaring someone righteous even though they are not, would logically also apply to sanctification, that is God could technically declare someone holy who is not, but Protestants realize this cannot be, since you cannot have a filthy holy object.
Even if one resists the above thoughts/conclusions, which I don't think is fair to resist them too stringently, the Old Testament Sacrifices were merely shadows of bigger realities to come (Heb 10:1), and thus the variety of OT sacrifices were all trying to point to something bigger in the future. This goes against the Protestant reading of the Mosaic Law, which is that they think it was able to save but sinfulness in humanity prevented them from keeping the Law perfectly. In reality, Hebrews 10:1 is saying the OT System never was meant to provide salvation, it was always merely a shadow. We even see the New Testament as less complicated system and more forgiving system, since the OT was multitude of sacrifices for this and that, especially small sins requiring major effort to atone for, and these Old ways never seemed to take Israelites to the next level in their spiritual life. When Christ came, all that Old stuff began to fade away, culminating with the Destruction of the Temple in 70AD, leaving merely the Cross and it's continual expression in the Eucharistic Offering. In fact, it would be impossible to suggest Hebrews was getting rid of all ritualistic worship with ministers, since even Protestants continue to offer the Eucharist at the hands of their pastor and elders.
Note, (here) is a good Catholic article on this from another blog that I read years ago but forgot about it until now.