Isn't it unjust to punish an innocent man for others' sins?
It would be, if he were forced. Jesus went willingly, as the judge himself taking the penalty. Isaiah foretold exactly that.
The Quran says no one bears another's burden. Isaiah says the LORD laid on his servant the iniquity of us all, and that the servant poured out his soul to death willingly. Love that pays someone else's debt is not injustice. It is grace.
164Say: Shall I seek another than Allah for Lord, when He is Lord of all things? Each soul earneth only on its own account, nor doth any laden bear another's load. Then unto your Lord is your return and He will tell you that wherein ye differed.
Quran 6:164 (Pickthall) · context
5But he was pierced for our transgressions. He was crushed for our iniquities. The punishment that brought our peace was on him; and by his wounds we are healed. 6All we like sheep have gone astray. Everyone has turned to his own way; and Yahweh has laid on him the iniquity of us all.
Isaiah 53:5-6 (WEB) · context
12Therefore I will give him a portion with the great. He will divide the plunder with the strong, because he poured out his soul to death and was counted with the transgressors; yet he bore the sins of many and made intercession for the transgressors.
Isaiah 53:12 (WEB) · context
25whom God sent to be an atoning sacrifice through faith in his blood, for a demonstration of his righteousness through the passing over of prior sins, in God’s forbearance; 26to demonstrate his righteousness at this present time, that he might himself be just and the justifier of him who has faith in Jesus.
Romans 3:25-26 (WEB) · context
18Because Christ also suffered for sins once, the righteous for the unrighteous, that he might bring you to God, being put to death in the flesh, but made alive in the Spirit,
1 Peter 3:18 (WEB) · context