At the heart of justification is the substitutionary work of Christ. This involves Christ's exchanging places with us. Given that our sin demands God's just wrath, Christ substitutionary work on our behalf involves Christ standing in our place and bearing the wrath that we rightfully deserve.
This is taught even in the Old Testament, as the prophet Isaiah foretold of the coming Messiah,
"But he was wounded for our transgressions;
he was crushed for our iniquities;
upon him was the chastisement that brought us peace,
and with his stripes we are healed.
All we like sheep have gone astray;
we have turned—every one—to his own way;
and the LORD has laid on him
the iniquity of us all."
This passage is clear that the Savior would take upon himself the iniquity of His people. This is exactly what Jesus did at the Cross. The Apostle Paul puts it this way in his letter to the Galatians,
"Christ redeemed us from the curse of the law by becoming a curse for us."
Because we have failed to follow God's law, we deserve the curse of not following it. What makes the Gospel good news is that Christ has bore the weight of sin that we deserve. He suffered the debt we owed. The just for the unjust, once and for all. We are able to be justified because Jesus substituted himself in our place.
We will see in the next observation that justification involves not only the substitutionary work of Christ in terms of him taking our sins, but it also involves the positive aspect of Christ imputing to His children His perfect righteousness.