Sin brings death and judgement
The Bible speaks plainly about sin's seriousness. Sin earns death, and Jesus describes a final judgement with everlasting consequences for those who reject God (Romans 6:23; Matthew 25:46). Since God is the giver of life, sin is not a small mistake. It is rebellion that breaks our fellowship with him.
Still, the Bible does not describe Jesus as a guilty person who needed to suffer forever for his own wrongdoing. He was innocent. His death was voluntary, given in love for people who could not rescue themselves.
For the wages of sin is death; but the gift of God is eternal life through Jesus Christ our Lord.
Jesus stood in our place
Isaiah foretold a servant who would be wounded because of others' transgressions and whose suffering would bring them peace. The passage says that the Lord laid the iniquity of us all on him (Isaiah 53:5-6). The New Testament sees this fulfilled in Jesus, who bore our sins in his body on the cross (1 Peter 2:24).
This does not mean Jesus became personally sinful. Peter calls him the just one who suffered for the unjust (1 Peter 3:18). Paul says that Christ took the curse resting on us (Galatians 3:13). At the cross, Jesus truly entered death's horror and bore the judgement sin deserves, while remaining the righteous Son who gave himself freely.
For Christ also hath once suffered for sins, the just for the unjust, that he might bring us to God, being put to death in the flesh, but quickened by the Spirit:
Why the sacrifice did not need to continue
It is understandable to ask how a finite time of suffering could address an eternal consequence. Scripture's answer is not a calculation of equal lengths of time. It points to the identity and work of Jesus. He is the sinless one, and his self-offering has a unique and sufficient value.
Hebrews contrasts the repeated sacrifices offered under the old covenant with Christ's sacrifice. Jesus appeared once to put away sin by offering himself, and he offered one sacrifice for sins for ever (Hebrews 9:26-28; Hebrews 10:12-14). His work was not unfinished because he did not remain dead or under judgement eternally. It was finished because he fully accomplished what was needed to deal with sin.
But this man, after he had offered one sacrifice for sins for ever, sat down on the right hand of God;
The resurrection matters
Jesus' resurrection does not make his suffering less real. It shows that death could not finally hold him. Peter says God raised Jesus from death because it was not possible for death to keep its grip on him (Acts 2:24).
The earliest Christian message joins these truths together: Christ died for our sins, was buried, and rose again (1 Corinthians 15:3-4). For those who trust him, his resurrection means sin and death do not get the final word.
Christians have used different pictures to explain how the cross works, including sacrifice, substitution, victory, and reconciliation. But they share this centre: Jesus gave himself completely, his sacrifice was sufficient, and he rose again. Reading the Gospel accounts alongside these passages can help this become more than an abstract answer.