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xyz said:

Christ died for all sins, past, present, future, so that no-one can claim impunity against God.

So then, no one has anything to worry about. If Christ has atoned for all the sins of all the people, then everyone is saved automatically whether they know it or not.

But the Bible clearly indicates that all are not saved, therefore we must conclude that Jesus did not atone for those people's sins. His sacrifice was certainly sufficient to do so, but His intention from before the foundation of the world was to atone only for the sins of the remnant called out from the fallen race of Adam.