Some variant of the phrase "Hearts must begin to think" is scattered throughout Rudolf Steiner's work, including his very late summary Anthroposphical Leading Thoughts - which are bracketed by this assertion*.
My understanding is that it Steiner meant that the divine destiny of modern Man is to become a thinker with 'the heart' primarily: that is, an intuitive thinker; and with the feeling that our thoughts are located in the chest.
And the 'must' comes in, because Steiner also predicted that 'Head thinking', intellect, the thinking that we feel is located behind the eyes - would decline.
Therefore, if Men failed to to be hearth-thinkers, failed to embrace our destiny; then we would after a while hardly be able to think at all.
I feel this is demonstrably the case, here-and-now, very strongly.
It has been building-up, or rather crumbling-down, through my life. By now, people simply can't think: by which I mean think for themselves (because there is no other kind of thinking).
What passes for thought is just channeling and parroting mass media talking-points which themselves are dictated by a Global Agenda. All over the world, people are discussing the same things, making the same points, 'appropriately' emoting to order.
There is no use of analysis, comparison, memory; no learning from experience. No discernment about sources. No memory of lies and betrayals. Just empty chambers, echoing noises...
It is as if almost-everybody has disengaged their thinking, switched-off their brains - or else become demented; yet absolutely refused to develop their intuition - their primary thinking of the true self...
The result is: the pitiful state of uncomprehending, directionless, passive helplessness that apparently afflicts almost everybody in the developed world!
It seems to be absolutely futile to try and get people to think. Whether they really can't think, or whether they simply refuse to think; the fact is that they Will Not think; and especially not the formally-educated and educationally-credentialled managerial and intellectual classes.
They all-but stopped brain-thinking from about half a century ago; they have since had many chances and inducements to start again - but the situation deteriorates, inexorably.
Now, the minds of Men are empty, they do not select, do not process, do not analyse, do not compare, do not discern.
Such Men must begin to think with their hearts; or else they have no hope and will kill themselves, whether directly or indirectly, from (understandable) despair.
And Satan will have won their souls.
*Heart thinking was brought to my attention by Stanley Messenger in a recorded lecture I have mentioned before.