Well, everyone has sinned, yes I am aware of that. But the people whom God had chosen since the time of Adam who died faithful to God's commandments and covenant with them shall receive the promise on Judgment Day... not right after their death. The same with the sinners who died before the time of Jesus will receive their punishment at the same Day... not right after their death.
But at the time of Jesus, God made an everlasting covenant with God's followers in Christ's time. This is a new covenant. New priesthood. New law. ... which also stipulate a new way of salvation... and this is through Christ.
But there was a universal law regarding judgment, which states that "one will not be punished because of other's sins"... This goes the same with Christ.
Christ cannot die for the sins of others. He will violate that universal law if He does so. That's why He has to build a church. and He considered this Church as His body. and anyone who will become a part of His body will be saved in Judgment Day.
Christ, being the Head, and the Church, His body - becomes a "one new man" as exposed in the letter of Paul to the Ephesians. This "one new man" really a ONE MAN in the sight of God... so Christ has to die, not for the sins of the whole world but for the sins of His body... ONLY His body.
But being a part of Christ's body, one should remain faithful and obedient to God's everlasting covenant.