A great quote from T. F. Torrance in his article, "The Hypostatic Union" (36, paragraphing mine) —
It is important to see that if the Deity of Christ is denied, then the Cross becomes a terrible monstrosity.
If Jesus Christ is man only and not also God, then we lose faith in God and man.
We lose faith in God because we could not believe in a God who allows the best man that ever lived to be hounded to death on the Cross—is that all that God cares about our humanity and its search after God, after truth and righteousness and peace?
Put Jesus Christ a man on the Cross, and put God in heaven, like some Mohammedan deity imprisoned in His own lonely abstract Deity—and you cannot believe in Him, in such a god who is monstrously unconcerned with our life, and who does not even lift a finger to help Jesus.
But if you deny the Deity of Christ we lose faith in man also, for that means that man is such that when he sees the very best, the very highest and noblest the world has ever known, he crucifies it in spite, and will have nothing to do with it except to hate it.
Put God in heaven, and Jesus a man only on the Cross, and you destroy all hope and trust, and preach a doctrine of the blackest and most abysmal despair.
Denial of the Deity of Christ destroys faith in God and in man, and turns the Cross into the bottomless pit of darkness.
But put God on the Cross, and the Cross becomes the world’s salvation.
All the Gospel rests upon the fact that it is God who became Incarnate, and it was God who in Christ has reconciled the world to Himself.