Although some opponents may interpret the clause in Hb 10:14 (“...made perfect forever those who are being made holy”) as suggesting that the salvation of the Christian is complete and totally secure with no possibility of falling away, this is not what the verse is teaching. We can see this by the way the word “perfect” is used in the book of Hebrews. According to Hb 10:1-2, the individual’s “perfection” refers to having his sins completely forgiven in order that the conscience may be free of guilt, something which the Old Covenant law could not provide (cf. 7:19; 9:9). Thus, the individual stands “perfect” because his past sins have been completely forgiven, not because he has reached a perfect state which eliminates the possibility of losing his state of grace. It follows, then, that the use of “perfect” here does not mean that the individual cannot retard the sanctification process, or that his eternal perfection is a foregone conclusion (Hb 11:40; 12:23). The verbal form chosen for “being sanctified” is a Greek participle of continuing action, which specifies the process of sanctification, a process by which we are continually forgiven of our sins, albeit now it is a complete or “perfect” forgiveness for the sins we have confessed. In other words, Christ did not make a blanket forgiveness of sin but has perfected the process by which sin is forgiven when it is confessed. Thus no more sacrifice is needed for past sins, but this does not mean we cannot forsake the process by refusing to repent of our future sins. Indeed, half the book of Hebrews shows how it is possible to fall from the sanctifying process, as for example, Hb 10:29: “How much more severely do you think a man deserves to be punished who has....treated as an unholy thing the blood of the covenant that sanctified him....” Notice that St. Paul teaches that the individual can fall from sanctification. (Robert A. Sungenis, Not By Bread Alone: The Biblical and Historical Evidence for the Eucharistic Sacrifice [2d ed.; State Line, Pa.: Catholic Apologetics International Publishing, Inc., 2009], 87-88)
The clause in Hb 10:14, τετελειωκεν εις το διηνεκες τους αγιαζομενους (literally: “he has perfected forever being sanctified”) uses the perfect indicative “τετελειωκεν” (“he has perfected”). St. Paul uses the concept of “perfect” quite often in Hebrews (cf. 2:10; 5:9; 6:1; 7:11, 19, 28; 9:11; 10:1, 14; 11:40; 12:2, 23). According to Hb 10:1-2, “perfection” for the individual refers to having one’s sins forgiven completely in order that the conscience may be free of guilt, something the Old Covenant law could not provide (cf. 7:19; 9:9). The joining of τετελειωκεν to the present passive participle “αγιαζομενους” (“are being sanctified”) shows that the completed act of Christ, which has been achieved and thus stands forever, is in a progressive, not a complete, relationship to us in sanctification (so Moulton’s Grammar). If St. Paul had meant to convey a complete and unalterable salvation, he could have implied this by following the main verb with the noun form τους αγιους (“the sanctified”); but he did not choose that form. (Ibid., 88 n. 89)
All the priests of the new covenant offer the same sacrifice continually, in every place and in every age; because the sacrifice which was offered for all is also one—the sacrifice of Christ our Lord, who accepted death for us, and by the offering of that sacrifice has purchased salvation for us, as blessed Paul says, “By one oblation,” he says, indeed, “He hath perfected for ever them that are sanctified (Heb. 10:14).’” (Theodore of Mopsuestia, Catechetical Homilies 19, in ibid., 250)