Verses 6-8 are quoted in Hebrews 10 as applying to the incarnation of Jesus (when he became a man). The author of Hebrews 10 is clear that it is Jesus who is speaking in the psalm, which means that in these verses of Psalm 40 we are allowed to listen in to a divine conversation between the Father and the Son just as the Son was about to come into our world. Yet it is a conversation given in the form of a song. The Son is singing to the Father a song of gladness and joy.
The first item in the song is that the Father took no pleasure in all the sacrifices that were offered in the Jewish ritual because they could not deal permanently with the problem of sin. Although he had given instructions about them, the Father was looking forward to the time when they would be abolished. And that time had drawn near. So the Son sings to the Father about a development that pleased him.
The second detail is found in the line, ‘My ears you have opened.’ The psalmist may be referring to the practice in Israel when a slave wished to show his total devotion to his master by having him bore through his ear to the doorpost (Exod. 21:6). If this is the meaning, it points to the amazing willingness of Jesus to dedicate himself to fulfil the Father’s will. Taking the practice of performing the boring at the door, we could say that Jesus allowed his ear to be bored through by the Father at the doorstep of heaven as he was about to enter this world.
The author of Hebrews did not quote from the Hebrew Old Testament when translating this phrase. Instead he cited the Septuagint rendering (the Greek Old Testament) which reads, ‘a body you have prepared for me’. The translators of the Septuagint interpreted the Hebrew clause when they translated it. Yet their interpretation was made under the supervision of the Holy Spirit and they provided the full meaning of the Psalmist’s original phrase, and their rendering was used by the author of Hebrews, that the reference was to the incarnation of the Messiah.
The clause in Hebrews 10 indicates that it was the Father who decided what the humanity of Jesus would be like. This statement is not in conflict with what the angel said to Mary in Luke 1:35: ‘The Holy Spirit will come upon you, and the power of the Most High will overshadow you; therefore the child to be born will be called holy—the Son of God.’ Rather it reveals that each person of the Trinity was involved in the Incarnation: the Father planned what the human nature of Jesus would be like, the Son assumed the human nature into permanent union with his divine person, and the Spirit formed the human nature in the womb of Mary. Of course, the Incarnation is a great mystery, but that is inevitable given that it was a divine activity.