“In Christ”

Sometimes when I hear people describing objective justification, they will say that "in Christ" the whole world is forgiven/justified. What does it mean to be 'in Christ' and how is the whole world "in Christ"?

The phrase “in Christ” explains the means by which God reconciled sinners to himself. He did that by means of his Son’s saving work in life and in death.

2 Corinthians 5:18-19 provides context for that phrase: “All this is from God, who reconciled us to himself through Christ and gave us the ministry of reconciliation: that God was reconciling the world to himself in Christ, not counting people’s sins against them. And he has committed to us the message of reconciliation.”