In Genesis chapter 3 God sovereignly appointed Adam to be all of mankind’s representative.
There was no death prior to Adam’s sin in Genesis 3. However, after he sinned death, entered the world.
Adam ate a piece of fruit from the one tree God said he couldn’t eat from, and because of that one act of rebellion God sentenced every human being to death.
It is completely just of God to appoint Adam as our representative. And it is by God’s grace that He has appointed Christ as a representative for all those would believe in Him.
Jesus and His redeeming work as our representative far exceeds Adam and his condemning act as our representative.
Christ is the true and better Adam because He brought us life instead of death.
In Christ’s active obedience, He perfectly obeyed God and His law at all points of His life.
In Christ’s passive obedience, He bore the full penalty that God demands of every sin and every sinner.