Cupitt's claim is that human life is "outsideless": that there is no truth deriving from an external reality, but rather "truths" which humans create for themselves.
This, in Cupitt's view, has the great benefit of leaving humans wholly autonomous rather than heteronomous (governed by external laws). Individuals don't need to measure themselves according to an external standard, but are free to choose without impediment who they are to be.
Bear in mind Cupitt's idea that we are "outsideless", self-creating beings as you read the following from Father James Schall:
The initial choice that each of us has to make in life is whether we think the world and ourselves already exist with some intelligible content to define what we are or whether there is nothing there but what we put there.
Conservatives will largely accept the first option, liberals like Cupitt are more likely to hold the second view.
Father Schall then suggests why the conservative option might seem less appealing than the liberal one:
The former position, it would seem, is rather demanding on us. It suggests that we are not our own self-creators, that what we are is something for us to discover, not make out of our own imaginary resources. But we are seemingly freer if there is nothing there in the first place, if we are solely responsible for our world and our own being.
So, if the liberal option appears to make us freer to be self-created, why not choose to adopt it? Father Schall makes an important observation:
The trouble with being so absolutely free that nothing is presupposed, however, is that what is finally put there is also only ourselves.
Father Schall goes on to develop this important observation as follows:
In such a view, everyone's world is identical, full of only themselves, with their own laws enforced by no one but themselves. On this premise, no reason can be found not to be something else tomorrow. A world full of nothing but Schall, it strikes me, as utterly boring. A world in which Schall is never the same is even worse.
Is there really nothing more to us that what our own subjective self puts there? What measure of substance does a "self" retain if it can change to any degree, in any direction, at any time?
Liberals think that by asserting a radically autonomous, fluid, multiple self that they are freeing the individual to be limitless. To the conservative mind, though, they are achieving the opposite: they are defining the individual in a way which contains the individual to a small and superficial existence.