Thinkers I respect such as Owen Barfield and Jeremy Naydler have stated that Rudolf Steiner is essential to our time. But given the truly vast volume of his work, its range, as the problematic nature of his legacy this leaves open the question of just what it is about Steiner that is essential? (In contrast with what is of perhaps important but lesser status: as well as what may better be neglected or ignored.)
Steiner himself gave a clue in the importance he attached to his earliest books, especially those about Goethe's science, his doctoral thesis published as Truth and Knowledge and (in particular) The Philosophy of Freedom. In other words, these were works of a metaphysical nature rather than being concerned with Steiner's specific or detailed 'findings'.
Steiner's modern legacy, by contrast (as far as I can determine) is focused around these specific findings - for example in relation to education, medicine and agriculture. There is also a considerable and commendable publishing and dissemination activity with respect to the vast number of works and the scholarship of summarising, systematising and analysing these works. Beyond this, there is the activity of teaching and supervising a specific technique of meditation.
My feeling is that none of this captures the essence and none is 'essential'.
(In one paragraph) What seems to me essential is what Steiner called monism and modern people might call holism - in particular, the bringing of imagination into the realm of one, single mode of thinking that can be called true 'science'; so that the perceptual ('objective', external) world of the natural sciences is united (again) with the conceptual ('subjective', inner) world of imagination; and this in a way that both heals our personal alienation and also creates a realm of public and shared discourse - a realm which is variously referred to by Steiner as Spiritual or Occult or Esoteric Science.
This making of a science of the imagination, and the possibility that each of us participate in it; requires, first, a proper understanding of the nature of science. This comes from the Goethe books - but has, I think, been lost underneath a focus on the Steiner-described and recommended technique of meditation as if this was the science.
As I argue in my book Not Even Trying
real science cannot be defined by technique but only by aim and ideals - science is, descriptively, nothing more specific than a truthful sustained examination of some rather specific class of phenomena, done in such a way that it is a social activity - which entails group production of some kind of public, communicable, evaluable content.
Thus, spiritual science cannot be captured by any particular technique of meditation, but only by the aim of some (perhaps small) group of investigators studying the imagination (presumably mostly by introspection, but in principle by any helpful method); honestly and in a sustained manner, and inter-communicating and critically evaluating their findings.
If this description of Steiner's essence is correct, then we can see that his essence is a mid-level activity, as appropriate to any science - it is not an ultimate activity like a religion.
This is confirmed by Steiner's biography - he was himself a Christian (albeit of a very unusual type) but did not require of his followers that they be Christian. But, and this is important, Steiner's philosophy presupposes religion of a certain type - it only makes sense within the metaphysical context of a religion that enables his work to have meaning.
Steiner's work does not make sense in a secular, atheistic context - with the nihilistic metaphysics that entails - because in such a context it is no more than a large number of bare assertions.
In sum, a true follower of Steiner must be religious (within a restricted range of deistic religions) - if he is not to be engaged in a self-refuting and ultimately incoherent activity; but he need not necessarily be Christian as Steiner himself was.
What makes Steiner essential (or, at least, nearly so) is that he uniquely offers the possibility of a Science (which modern Man seems to require) of the Imagination (which Modern Man to desperately lacks); and as an active, fundamentally-engaged participant (not merely as a passive observer, consumer or obedient follower).
In this sense, therefore, the essence of Steiner is as necessary for modern Christians and other religious people as it is for the secular majority.