The answer is Propitiation…Jesus bore the wrath of God and punishment of all the sinners who would ever turn to him in repentance and faith.
How should we explain Jesus’ belief in the necessity of his death? How should we account for the fact that what drove him on throughout his public ministry, as all four Gospels testify, was the conviction that he had to be killed? And how should we explain the fact that, whereas martyrs like Stephen faced death with joy, and even Socrates, the pagan philosopher, drank his hemlock and died without tremor, Jesus, the perfect servant of God, who had never before showed the least fear of man or pain or loss, manifested in Gethsemane what looked like blue funk, and on the cross declared himself God-forsaken? “Never man feared death like this man,” commented Luther. Why? What did it mean?
…if we relate the facts in question to the apostolic teaching about propitiation, all becomes plain at once. “May we not urge,” asks James Denny, “that these experiences of deadly fear and of desertion are of one piece with the fact that in his death and in the agony of the garden through which he accepted that death as the cup which his Father gave him to drink, Jesus was taking upon him the burden of the world’s sin, consenting to be, and actually being, numbered with the transgressors?” (The Death of Christ, 1911 ed., p. 46).
Had Paul or John been asked this question, there is no doubt what they would have answered. It was because Jesus was to be made sin, and bear God’s judgment on sin, that he trembled in the garden, and because he was actually bearing that judgment that he declared himself forsaken of God on the cross. The driving force in Jesus’ life was his resolve to be “obedient to death–even death on a cross” (Phil 2:8), and the unique dreadfulness of his death lies in the fact that he tasted on Calvary the wrath of God which was our due, so making propitiation for our sins.
Centuries before, Isaiah had spelled it out: “We considered him striken by God….The punishment that brought us peace was upon him….The LORD has laid on him the inquity of us all….For the transgression of my people he was stricken….It was the LORD’s will to crush him…the LORD makes his life a guilt offering” (Is 53:4-10).
Taken from Knowing God by J. I. Packer, pp. 192-194.