Only a sinless human being could satisfy God's justice, and that would be for himself alone. Only God can stand on behalf on all sinful people. God's mercy and love demanded the rescue of all people.
So God the Son, with the power to represent all people, became mortal, fully human, with the power to satisfy God's justice.
Salvation is the free gift of perfection in God's Kingdom for those who accept the death of Jesus Christ as their own. If we die, we cannot satisfy the requirements for the Kingdom. If we ive, justice awaits. Jesus Christ took our punishments upon Himself so that we can die to sin and live a new life in Him.
Other posts in this series:
2 The Kingdom of God
5 The Old Man
Inventing the Individual: Book Review
2 months ago