This passage is about [true] faith as opposed to unbelief or erroneous faith. I've substituted "faith" for "it" in a couple places.
True faith does NOT
arbitrarily choose objects to set up signs, in that way inventing knowledge of God at its own good pleasure. Faith knows God by means of the objects chosen by God Himself. It recognizes and acknowledges God's choice..and uses these special works of God as they ought to be used.
This is by the author of the work that "fell like a bombshell on the playground of the theologians." A scholar steeped in 19th century German liberalism who STILL believed in the simple things of faith. "Jesus loves me, this I know, for the Bible tells me so!" Karl Barth really summarized his life's work with a child's song, in answer to a perhaps impertinent question from an American audience. Coincidentally, Carl Barth was my great grandfather! Barththe American farmer, not the Swiss/German theologian ;-)
Barth also has some great thoughts about how Jesus placed Himself and His coming above marriage and possessions. Without condemning either, Jesus shattered our faith in even the most godly of institutions, either of which can easily become an idol. And Barth wasted little time in speculating about the "authenticity" of particular sayings of Jesus. How Rudolf Bultmann wasted his genius on such foolishness will be, perhaps, forever beyond my comprehension. If I, like Bultmann, had stood by in relative safety during the Holocaust, I might have spent the rest of my life atoning for my inaction, instead of undermining the very foundation of Christian theology, the inspired, Holy Scriptures. Oddly, there is still some devotional value in Bultmann, if you skip over the speculative parts of his writings