Barth's concern in this section is to shore up the defenses of the fortress-like wall that defends the absolute freedom of God to speak ubi et quando visum est Deo (where and when it pleases God). Barth refers to God's Word in this section as God's mystery. That means that it cannot be defined, cannot be roped in and corralled, cannot be delimited or cordoned off and controlled by human understanding. No, the Word of God remains free.
So, even though in previous sections, Barth has spoken about God's Word as "speech" and "act," he is careful now to say, "God's speech is different from all other speech and God's action is different from all other action." (CD I.1 §5.4 p. 164).
There are three ways that God's Word is veiled in mystery. 1) God's Word always comes in the veil of the secular, by which I believe he means "worldly." We only experience God's word through this world, which by definition, can never be the pure word of God. According to Barth, the pure, unveiled Word of God would be "the end of us." 2) God's Word comes to us in its one-sidedness. Now this was completely incomprehensible to me, since for the remainder of this sub-section, Barth refers to the Word always coming in two sides: veiled and unveiled, veiled in its unveiling, and unveiled in its veiling. God's word is an antithesis, it comes in contradiction to itself, it reveals as it conceals. 3) Finally, God's word is mystery in its spirituality, that is, in its coming through the Holy Spirit. Only through the Holy Spirit is God's Word heard and received in faith. The consequence of this for Barth is that faith is a gift, it cannot be mustered up, worked for, prayed for, etc. Faith is not a "work" that a human performs, but a gift that a human receives.
Once again, Barth is trying to protect the freedom of God's word, as if it needed protecting. But, what he is really guarding against, or warning against, is human pride that would think it had grasped the Word of God. I will end with the money quote from Barth on this issue:
"One cannot lay down conditions which, if observed, guarantee hearing of the Word. There is no method by which revelation can be made revelation that is actually received, no method of scriptural exegesis which is truly pneumatic, i.e., which articulates the witness to the revelation in the Bible and to that degree really introduces the Pneuma, and above all no method of living, rousing proclamation that truly comes home to the hearers in an ultimate sense. There is nothing of this kind because God's Word is a mystery in the sense that it truly strikes us spiritually, i.e., in all circumstances only through the Holy Spirit." (CD I.1. §5.4 p. 183).