Text in red are my additions to the commentary.
4. For it is impossible that with the blood of oxen and goats sin should be taken away.
The invalidity of the Old Law sacrifices to remit sins is evident from their very nature, for there is no proportion between them and the result sought. It is impossible that the blood of irrational animals of itself should be able to cleanse the guilty conscience of an intelligent and free creature. The sins that were actually forgiven under the Old Law were remitted through the power of the blood of Christ which those ancient rites prefigured. It was only as symbols of Christ’s sacrifice, therefore, that the Levitical sacrifices had any real validity.
In the preceding verses the author has been repeating in other words arguments already given to show the failure of the Jewish sacrifices. Now (in the following verses) he introduces a new and more powerful argument based on Psalm 40:7-9 according to the LXX (Ps 39:7-9 in some translations). From that text he proves that centuries before the coming of Christ it was announced that God was not pleased with the ancient sacrifices, and that they would be superseded by the perfect sacrifice of the will of Christ. The Psalm is certainly Messianic, and the verses here quoted represent the Messiah as saying at the moment of His incarnation that all the ancient sacrifices were unpleasing to God, because inadequate for human needs, and that consequently a body had been especially prepared for Him by God the Father which should be the organ and instrument of a sacrifice of perfect obedience and absolute submission of His will to the divine will. Such a sacrifice would be worthy of God, since it was the sacrifice of the Son of God, and sufficient atonement for man’s sins.
5. Wherefore when he cometh into the world, he saith; Sacrifice and oblation thou wouldest not: but a body thou hast fitted to me:
Wherefore, i.e., because of the imperfections of the Jewish sacrifices.
When he cometh into the world, i.e., when the Messiah became incarnate.
Sacrifice and oblation, i.e., bloody and unbloody offerings.
Thou wouldst not, i.e.. Thou didst not desire, because of their inefficacy.
A body thou hast fitted for me. So the LXX; the Hebrew has: “Ears thou hast digged (i.e., opened) for me,” so that the will of the Father might be readily perceived and obeyed. According to the LXX rendering, the body has been prepared to act, to carry out the behests of the divine will. Hence, the underlying meaning is the same in both translations.
6. Holocausts for sin did not please thee. (see note below).
Holocausts were bloody offerings, all of which were entirely consumed by fire on the altar.
Sin-offerings. Our Vulgate has missed this correct rendering of the Hebrew and the Greek. Thus, the terms here employed cover the whole range of Jewish sacrifices, with none of which was the divine will pleased; God wanted a complete obedience and an entire spiritual consecration.
Note the translation of verse 6 reads: Holocausts for sin… the Greek reads: ολοκαυτωματα και περι αμαρτιας. The Greek construction seems to be a bit ambiguous, for the ASV translates: In whole burnt offerings and sacrifices for sin;… and the KJV translate: In burnt offerings and sacrifices for sin…in both translations the word sacrifices is in italics. Strangely, the Douay-Rheims translation of Psalm 40:7 reads: Burnt offering and sin offering thou didst not require. Why that was not reproduced here is unknown to me.
7. Then said I: Behold I come (in the head of the book it is written of me), that I should do thy will, God.
Then said I, etc. Having understood the divine will, the Messiah replies that He is ready to do it, that is, He is prepared to sacrifice Himself, to consecrate His life to complete obedience in accordance with prophecy, for the whole Old Testament speaks of His advent.
The head of the book. Rather, “the roll of the book,” i.e., the entire Old Testament regarded as a book of prophecy about the Messiah. The Old Testament was written on strips of parchment or vellum, which were then wound about a roller, and the whole book was called a roll. This phrase here, “the head of the book, etc.,” is to be read as a parenthesis.
8. In saying above: Sacrifices, and oblations, and holocausts and sin offerings thou wouldcst not, neither are they pleasing to thee, which are offered according to the law.
9. Then said I: Behold, I come to do thy will, God. He taketh away the first that he may establish that which followeth.
10. In the which will we are sanctified by the oblation of the body of Jesus Christ once.
8-10. The Apostle makes application of the words just quoted from the Psalmist, saying that according to those words of prophecy the ancient sacrifices which were offered as prescribed by the Law have been superseded by the new and perfect sacrifice which Christ has offered to the Father, and that we, by virtue of that perfect sacrifice of Christ, have been “sanctified,” i.e., dedicated to God.What Christ did and endured for us in the days of His flesh has merited for us this sanctification and dedication.
The argument developed here in verses 5-10 must have powerfully influenced the readers of the Epistle who were wavering in their Christian faith; for it showed that by the mouth of the Psalmist the Old Testament itself condemned the Jewish sacrificial system, foretold its abrogation, and prophesied the perfect sacrifice of Christ and the establishment of the new type of religion introduced by Him. With such evidence drawn from the Old Law itself, how could any Jewish Christian think of forsaking Christianity and lapsing into Judiasm?