The answer is the word "substitute." Christ was the unviversal substitute, and only God Himself could choose Himself as the substitute for absolute just retribution for sin. It is necessary that the the consequences of sin be born by those who commit them, and nothing can undo the sin that has been committed. But it is God who must execute judgment and meet out retribution, and only God who could supply the appropriate substitute for that retribution. This he did, by becoming a man, living a sinless life, and suffering and dying on the Cross, as a condemned criminal. God the Father did not punish God the Son, God the Father allowed God the Son to be condemned and executed by men, as a substitute for all mankind.
The language of Isaiah 53 is highly metaphorical, and I readily admit expressions like, "it pleased the Lord to bruise him," and, "he hath borne our griefs and carried our sorrows," can be construed as pictures of punishment and payment. But the plain fact is, we know they are metaphors, because the Father did not bruise the son, and we all have griefs and sorrows and sickness. (Now if Jesus had literally borne them for us, we should not have them.)
Everywhere the Bible talks about the forgiveness of God, but to be just, sin cannot just be pardoned, as though there were no consequences for sin. There must be some basis for that pardon. The atonement secured universal pardon for all mankind, and forgiveness is granted to all those who accept that pardon.
There is a condition, however. One cannot remain a rebel and enemy in one's heart and mind and be still eligible for the free gift. The gift is offered to all those who willing throw down the arms of their rebellion, and embrace the forgiveness extended to all those who choose the reconciation offered in Christ.
Finally, if the atonement were based on Christ's suffering and death paying for sin or bearing the exact punishment for the sin of all men, forgiveness would not be required. Debt that has been paid does not need to be forgiven. Since the Bible clearly teaches our sin has been forgiven, the atonement must be the just basis for that forgiveness, not payment for it.
Since you believe the atonement was unlimited, if it were literally the exact bearing of the punishment for all mankind's sin, to send anyone to hell would be extracting the same punishment twice, since Christ would already have born the punishment once.
However, much about the atonement, I beleive, remains a mystery. That Christ's death has secured forgiveness for our sins, and is the only way we can be reconciled with God is certain. The views I have expressed are my own based on Scripture alone. I am not pressing anyone else to accept them. I certainly do not accept the teachings of other men, especailly those of Augustine, by way of Calvin, Wesley, Luther, or anyone else.
Hank