Spiritual science can only arise on earth. Nowhere else can it be learned. It is a revelation of the supersensible world but in such a form as can arise only here on earth.
One might well say that supposing all this to be true, surely one could be instructed about the supersensible in the spiritual world in a form other than that of spiritual science! One can certainly think this, but it is not true. For if a person is at all predisposed to gain a genuine connection with higher worlds, he can do so only by means of spiritual science. If he fails to draw near to spiritual science or anthroposophy on earth, no other form of existence will help him to get to know it. But no other form of existence will help him to gain a genuine human connection with the supersensible worlds, either.
This need not plunge us into despair about the many people who are still refusing to know anything about anthroposophy. They will return and establish a connection with it at a later stage.
Source: Rudolf Steiner – GA 140 – Life Between Death and Rebirth – XI: The Mission of Earthly Life as a Transitional Stage for the Beyond – Frankfurt, 2nd March 1913
The great Rudolf Steiner Quotes Site 