All our sins are our own, but for salvation purposes, as Paul says, God
“calleth the things that are not, as though they were.” (Romans 8:17)
And, it is Paul who also wrote:
“through the one man’s disobedience the many were made sinners.” (Romans 5:19)
In context, “the many” is in reference to all of Adam’s descendants, in contrast with Adam, who is only one person. In God’s sight, the many were made sinners through one man, so that
“through the obedience of one man the many will be justified.”
While justification [being made straight, righteous] is now reckoned through faith in the sacrificed blood of Jesus, justification will, in the age to come, be reckoned, or counted, to all who died while unjustified, otherwise there would be no resurrection of the unjust; there would be no resurection of judgment. (John 5:28,29; Acts 24:15) All unbelievers in this age would remain dead for all eternity. However, because the sacrifice of Jesus to God for the sin of world provides the price to ‘ransom all from the power of sheol,” and ‘redeem all from death’ (Hosea 13:14), not just those justified by faith in this age will be raised in the last day, but all the dead in the sea, death and hades (sheol of Hosea 13:14) will be raised. (Revelation 20:13) In this way, Jesus died for the sins, not just for those who believe in this age, but the whole world, thus releasing them from the condemnation of death, so that they may be raised in the last day. — John 1:19; 4:42; 12:47,48; 1 John 2:2; 4:14.
If, however, the final judgment upon mankind is due to each individual’s sin in this age, this would mean that the one sinless man, Christ Jesus, could in no way satisfy God’s justice, for every man would be under a separate condemnation for his own sin, and thus if there have been 40 billions sinners, there would have to be 40 billion non-sinners to sacrifice themselves to offset the wages of sin for those 40 billion sinners. By condemning all in one man, only one sinless man is needed to provide a sacrifice to God so that God “would be just and the justifier of the one who has faith in Jesus.” — Romans 3:26.