What else could have filled Christ with such dread than the knowledge that in his suffering he would endure in the place of sinners the wrath of God? This is what the "cup" that he spoke of stood for. I read the following from Jonathan Edwards a few years back and found it helped me tremendously to reflect on the atonement:
"The strength of Christ's love more especially appears in this, that when he had such a full view of the dreadfulness of the cup that he was to drink, that so amazed him, he would notwithstanding even then take it up, and drink it. Then seems to have been the greatest and most peculiar trial of the strength of the love of Christ, when God set down the bitter portion before him, and let him see what he had to drink, if he persisted in his love to sinners; and brought him to the mouth of the furnace that he might see its fierceness, and have a full view of it, and have time then to consider whether he would go in and suffer the flames of this furnace for such unworthy creatures, or not.
This was as it were proposing it to Christ's last consideration what he would do; as much as if it had then been said to him, 'Here is the cup that you are to drink, unless you will give up your undertaking for sinners, and even leave them to perish as they deserve. Will you take this cup, and drink it for them, or not? There is the furnace into which you are to be cast, if they are to be saved; either they must perish, or you must endure this for them. There you see how terrible the heat of the furnace is; you see what pain and anguish you must endure on the morrow, unless you give up the cause of sinners. What will you do? is your love such that you will go on? Will you cast yourself into this dreadful furnace of wrath?' Christ's soul was overwhelmed with the thought; his feeble human nature shrunk at the dismal sight. It put him into this dreadful agony which you have heard described; but his love to sinners held out.
Christ would not undergo these sufferings needlessly, if sinners could be saved without. If there was not an absolute necessity of his suffering them in order to their salvation, he desired that the cup might pass from him. But if sinners, on whom he had set his love, could not, agreeably to the will of God, be saved without his drinking it, he chose that the will of God should be done. He chose to go on and endure the suffering, awful as it appeared to him. And this was his final conclusion, after the dismal conflict of his poor feeble human nature, after he had had the cup in view, and for at least the space of one hour, had seen how amazing it was. Still he finally resolved that he would bear it, rather than those poor sinners whom he had loved from all eternity should perish.
When the dreadful cup was before him, he did not say within himself, why should I, who am so great and glorious a person, infinitely more honourable than all the angels of heaven, Why should I go to plunge myself into such dreadful, amazing torments for worthless wretched worms that cannot be profitable to God, or me, and that deserve to be hated by me, and not to be loved? Why should I, who have been living from all eternity in the enjoyment of the Father's love, go to cast myself into such a furnace for them that never can requite me for it? Why should I yield myself to be thus crushed by the weight of divine wrath, for them who have no love to me, and are my enemies? they do not deserve any union with me, and never did, and never will do, any thing to recommend themselves to me. What shall I be the richer for having saved a number of miserable haters of God and me, who deserve to have divine justice glorified in their destruction?
Such, however, was not the language of Christ's heart, in these circumstances; but on the contrary, his love held out, and he resolved even then, in the midst of his agony, to yield himself up to the will of God, and to take the cup and drink it."
From Jonathan Edwards sermon on Christ's Agony (Luke 22)