It's easy, because it's self-contradictory. If theology is "temporary", then even this statement about theology being "temporary" is itself temporary. One must make a non-temporary theological statement in order to state that all theology is temporary, just as one must make a universal and transcendent theological statement in order to say that theology is not universal and trascendent.
Take a look at these statement, from page 114...
...emergents reject metaphors like "pin it down", "in a nutshell", "sum it up", and "boil it down" when speaking of God and God's Kingdom, for it simply can't be done.
But even in saying those things, is he not "pinning down" things about theology? Even in saying that God and God's Kingdom cannot be "pinned down", does that not "pin down" something about God and God's Kingdom?
Now, one question that could be asked is, how far do they or would they go with this idea? For example, what about the fairly simple statement "God is love". Does that statement "pin down" God and our ideas of God? What if someone says "God is not love"? Is that statement as equally valid as the first?
If all theology is temporary, then what can we say that we really know about God? Can we even say "God is love" if we do not, cannot, have some sort of real non-temporary idea of what that means? If everything in our theology is only temporary, then how can we even say that there is a God? Because isn't saying "There is a God" a theological statement? And if that is so, then isn't it also a temporary statement?
Now, why is theology being temporary such a big characteristic of Emergent "theology"?
Because then, theology can be changed. If the Bible forbids something, like homosexuality, then if theology is temporary, then the theology in the Bible is temporary, Thus, one can posit that despite what the Bible says, things change, including theology, and that now we can think up reasons for saying that homosexual sex is now not forbidden.
Or, what about issues about beliefs and how they relate to salvation? While Jones does say that he believes in the Trinity and in Jesus being fully human and fully divine, he and other Emergents seem to consider someone like Marcus Borg to be a Christian, too. This is the same Borg who said that he would bet his life that Jesus did not rise from the dead, which belief would disqualify him biblically from any consideration as being a Christian. Did not Paul say that if Christ was not raised, our faith is in vain, and we are still in our sins? How, then, can one who teaches the Christ was not raised by considered a brother?
But if theology is temporary, then Paul's statement was maybe temporary, and we are not obliged to take it seriously today (though who knows what tomorrow will allow)? And so, is Borg, whose beliefs are miles outside of any measure of orthodox biblical Christian doctrine,welcomed with open arms by these "temporary" theology people?