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.
