I appreciate where Steiner -- George, not Rudolf -- is coming from with this line of thinking.
The unattainable dimensions of thought to which Steiner alludes are most glaring in the arts, particularly poetry. I have this personal theory that the novel maximized the creative space between man and God, and it did so in a positive rather than a negative sense – at least for a while. This no longer holds true for the most part.
At the same time, this creative chasm between man and God proved too great for poetry. Though some good poetry still existed in the mid-twentieth century, the creative space-separation became too strained to be tenable, and poetry basically collapsed.
The arts aside, unattainable dimensions of thought and creativity are now ubiquitous in all areas of human life. Ironically enough, these are most pronounced in spiritual and religious thinking, which virtually lack all semblance of creativity today.
I would argue that the "certain" dimensions of thought and creativity Steiner refers to depend on much more than God being a tenable supposition or the deep feeling (or not feeling) of God's absence as an overwhelming weight.
The dimensions of thought and creativity Steiner mentions are attainable once man understands that the overwhelming absence pressing down on Creation emanates from himself, not God.
And this absence can only be filled by man making the unattainable attainable via thinking and creativity.
God is there. Man isn't.
God is waiting, patiently for a creative answer to His call. Man continues to insist on being absent in Creation.