To strip the atonement of its substitutionary nature, as Rob Bell does, is to strip it of its power. It is to take the "good" out of the good news. It is to rip the heart out of the Gospel and therefore the hope out of human hearts. It is an act of profound cruelty for it robs the sinner of what he truly needs: a guilt-bearing substitute.
“There’s nothing wrong with talking and singing about how the ‘Blood will never lose its power’ and ‘Nothing but the blood will save us.' Those are powerful metaphors. But we don’t live any longer in a culture in which people offer animal sacrifices to the gods.
“People did live that way for thousands of years, and there are pockets of primitive cultures around the world that do continue to understand sin, guilt, and atonement in those ways. But most of us don’t. What the first Christians did was look around them and put the Jesus story in language their listeners would understand.”
- Rob Bell on the atonement from Love Wins
This is nothing new. Theological liberals have been diluting and even neutering the gospel for generations. In the 1920's J. Gresham Machen saw this clearly.
They [liberal preachers] speak with disgust of those who believe ‘that the blood of our Lord, shed in a substitutionary death, placates an alienated Deity and makes possible welcome for the returning sinner.’ Against the doctrine of the Cross they use every weapon of caricature and vilification. Thus they pour out their scorn upon a thing so holy and so precious that in the presence of it the Christian heart melts in gratitude too deep for words. It never seems to occur to modern liberals that in deriding the Christian doctrine of the cross, they are trampling upon human hearts. (Christianity and Liberalism, 120 [pagination may differ])