Colby Martin is a self-proclaimed "progressive Christian." I read his book - The Shift: Surviving and Thriving After Moving from Conservative to Progressive Christianity.
In this book Martin states his beliefs about Christianity. One of his beliefs is that "believing" is toxic. He writes:
“There does not exist one single way to be a progressive Christian; therefore the following pages won’t tell you what you need to do (or worse, what you need to believe) in order to become one.”
Note that in this logically incoherent sentence Martin states this belief: There does not exist one single way to be a progressive Christian. (Call this Belief 1.)
Apparently, to be a progressive Christian one must, or should, or ought to, believe in Martin's belief.
But, according to Martin, that's horrific ("worse"), since Martin is giving us a singular belief, about what we need to believe, in order to become a progressive Christian. ("Thou must believe there are many beliefs, and this is one you ought to believe.")
If someone does not believe in Martin's dogmatic belief, then it seems the unbeliever believes this: There does exist one single way to be a progressive Christian. (Call this Belief 2.)
But no true progressive Christian believes in Belief 2. Meaning that, to become a progressive Christian one must (or eventually will) believe in Belief 1. Which leads to Belief 3: There is no single way to become a progressive Christian, and to become a progressive Christian one must (or eventually will) believe this.
Beliefs are unavoidable.
Progressive Christianity has its own dogmatic beliefs. (On this, see theologian Michael Kruger, The Ten Commandments of Progressive Christianity.)
(Note: This is why atheists Richard Dawkins and Steven Pinker abhor postmodern illogic. See Pinker, Rationality: What It Is. Why It Seems Scarce. Why It Matters.)