Because he is Holy!
“And one cried unto another, and said, Holy, holy, holy, is the LORD of hosts: the whole earth is full of his glory. (Isaiah 6:3)”
God is eternal, he has always been “Holy”; he didn’t evolve over a space of time and eventually become holy, he is eternally Holy.
Holiness is his eternal nature; there will never be a moment where God will compromise his holiness. No matter what you think about your relationship with God, he will never love you so much that he will violate his position of holiness to accommodate your Sin; to do so would destroy who and what God is and he would become no better than any false God.
It is God’s unwillingness to compromise that allows God to be trusted always to honor his word; his nature, his character, his word is immutable; never-changing.
Since the Garden of Eden, the problem between man and God, is God’s holiness. God’s holiness says man must die since man is not holy, nor can man become holy by his own hand. However, God’s Mercy has devised a plan that will satisfy and appease God’s holiness.
This plan was put in place before God ever started his creation; according to the Book of Revelation (Rev13:8), Jesus is the lamb slain before the foundation of the world. Therefore, this plan (Jesus) is not a plan “B” or some other compromise; but rather it is the original plan God devised through his holiness and mercy, before the beginning ever began.
What happened in the Garden of Eden was no minor infraction, but rather it was a violation against God’s Holiness and the penalty of this violation is death; this is why throughout the scriptures there is references to “blood”. Blood represents life, the absence of blood causes death; Jesus didn’t give just a part of his life or a few drops of blood, he had to give it all for God’s plan to succeed.
“In whom also we have obtained an inheritance, being predestinated according to the purpose of him who worketh all things after the counsel of his own will: (Ehesians 1:11)”
God’s Mercy and God’s Holiness had a reconciliation meeting; in this counsel meeting God’s mercy put in place Jesus as the plan to reconcile man to God’s holiness, which allowed God to create man with free will. Without this plan in place, then God’s creation would have to be destroyed just as soon as man’s free will violated God’s holiness.
Reconciliation goes much deeper than just the need of forgiveness; man lost something inside of himself that only God can restore, that being the in dwelling presence of God’s Holy Spirit.
God didn’t want robots who mindlessly follow him, he wanted his creation to be able to love him back. Love requires the expression of free will; but with free will, comes the ability to also choose to reject God or fall into rebellion to God’s will and then come death or the consequences of that choice.
God’s plan was that he himself would become a man, (as a man he would take on the penalty for mankind, thereby he would as the “God-man”, he would then be able to also redeem all who will enter in to his plan.) It was necessary for Jesus not only be a man; but in order to redeem mankind, he also would have to have the purchasing power to redeem us all, which only the creator himself would have.
The payment for all of mankind’s sin would not come cheap, nor could just any kind of payment heal the breach between man and the holiness of God; the only one who could possibly make this payment, was God himself. If any man is found outside of this plan of God’s, then he would have to face God’s holiness alone and bear the penalty of eternal death.
When anyone asks me, why is belief in Jesus necessary, I am obligated to start out and say, “It because God is Holy!”