Should God, of his free choice, wish not to reveal himself but to remain silent, man would attain the ultimate and highest self-perfection of his spiritual and religious existence by listening to the silence of God. (Karl Rahner, Hearers of the Word, pg. 16)
Rahner’s point here is to emphasize that finding the conditions of possibility for humankind receiving revelation (the task of “philosophy of religion”) is separate from the question of whether revelation has “empirically” happened (if it has, it is the subject matter of “theology”). Elsewhere, he lines up the speech or silence of God as two seemingly parallel possibilities. But does this really make sense in his scheme? After all, Rahner determines that the “place” where we receive revelation is in human historicity and specifically in human language — hence we can expect revelation to have a determinate content and to have taken place at a determinate time (for example, the content could be Jesus’ proclamation of the Kingdom, and the time could be around 30 A.D. — for example).
If the Anknupfungspunkt were something like the pineal gland, then it would make sense for it to remain idle — but if the “pineal gland” here is the existential structure of humanity, then “listening to the silence of God” doesn’t seem to be a coherent possibility. God would somehow have to let us know, at a determinate time, that he intended to remain silent — as it were, to open his mouth to speak, then think better of it. Silence as sheer silence would not be revelation at all. The highest calling of humanity, in the event of a “null” revelation, would be to live as though there were no God — something the human beings in such a hypothetical situation would presumably do quite naively, without knowing or being able to know that they were doing so.
Or is Rahner thinking that at some point, someone would begin to feel awkward and ask his neighbor, “Do you feel like someone’s mad at you? Like there’s someone with their back turned, their arms crossed, really wanting you to notice that they’re not paying attention to you?” The “silence” of the null revelation would be the sound of God tapping his foot, furiously waiting for someone to ask him what’s wrong.