5. Christ's voluntary self offering, in contrast to those inefficient sacrifices, is shown to fulfill perfectly "the will of God" as to our redemption, by completely atoning "for (our) sins."
      Wherefore--seeing that a nobler than animal sacrifices was needed to "take away sins."
      when he cometh--Greek, "coming." The time referred to is the period before His entrance into the world, when the inefficiency of animal sacrifices for expiation had been proved [THOLUCK]. Or, the time is that between Jesus' first dawning of reason as a child, and the beginning of His public ministry, during which, being ripened in human resolution, He was intently devoting Himself to the doing of His Father's will [ALFORD]. But the time of "coming" is present; not "when He had come," but "when coming into the world"; so, in order to accord with ALFORD'S view, "the world" must mean His PUBLIC ministry: when coming, or about to come, into public. The Greek verbs are in the past: "sacrifice . . . Thou didst not wish, but a body Thou didst prepare for Me"; and, "Lo, I am come." Therefore, in order to harmonize these times, the present coming, or about to come, with the past, "A body Thou didst prepare for Me," we must either explain as ALFORD, or else, if we take the period to be before His actual arrival in the world (the earth) or incarnation, we must explain the past tenses to refer to God's purpose, which speaks of what He designed from eternity as though it were already fulfilled. "A body Thou didst prepare in Thy eternal counsel." This seems to me more likely than explaining "coming into the world," "coming into public," or entering on His public ministry. David, in the fortieth Psalm (here quoted), reviews his past troubles and God's having delivered him from them, and his consequent desire to render willing obedience to God as more acceptable than sacrifices; but the Spirit puts into his mouth language finding its partial application to David, and its full realization only in the divine Son of David. "The more any son of man approaches the incarnate Son of God in position, or office, or individual spiritual experience, the more directly may his holy breathings in the power of Christ's Spirit be taken as utterances of Christ Himself. Of all men, the prophet-king of Israel resembled and foreshadowed Him the most" [ALFORD].
      a body hast thou prepared me--Greek, "Thou didst fit for Me a body." "In Thy counsels Thou didst determine to make for Me a body, to be given up to death as a sacrificial victim" [WAHL]. In the Hebrew, Ps 40:6, it is "mine ears hast thou opened," or "dug." Perhaps this alludes to the custom of boring the ear of a slave who volunteers to remain under his master when he might be free. Christ's assuming a human body, in obedience to the Father's will, in order to die the death of a slave (Heb 2:14), was virtually the same act of voluntary submission to service as that of a slave suffering his ear to be bored by his master. His willing obedience to the Father's will is what is dwelt on as giving especial virtue to His sacrifice (Heb 10:7, 9, 10). The preparing, or fitting of a body for Him, is not with a view to His mere incarnation, but to His expiatory sacrifice (Heb 10:10), as the contrast to "sacrifice and offering" requires; compare also Ro 7:4; Eph 2:16; Col 1:22. More probably "opened mine ears" means opened mine inward ear, so as to be attentively obedient to what God wills me to do, namely, to assume the body He has prepared for me for my sacrifice, so Job 33:16, Margin; Job 36:10 (doubtless the boring of a slave's "ear" was the symbol of such willing obedience); Isa 50:5, "The Lord God hath opened mine ear," that is, made me obediently attentive as a slave to his master. Others somewhat similarly explain, "Mine ears hast thou digged," or "fashioned," not with allusion to Ex 21:6, but to the true office of the ear--a willing, submissive attention to the voice of God (Isa 50:4, 5). The forming of the ear implies the preparation of the body, that is, the incarnation; this secondary idea, really in the Hebrew, though less prominent, is the one which Paul uses for his argument. In either explanation the idea of Christ taking on Him the form, and becoming obedient as a servant, is implied. As He assumed a body in which to make His self-sacrifice, so ought we present our bodies a living sacrifice (Ro 12:1).

JFB.