As many worlds exist, as there are organs able to perceive them, and this is an infinite number of worlds! We are unable to perceive them to-day, because we still lack the organs of perception. […] These worlds are still inaccessible to us yet they exist, — they exist, where we ourselves exist. The only thing needed is that our eyes should be opened, for these worlds are in our very midst.
The words of Christ: “Do not seek the Kingdom of God, for the Kingdom of God is in your midst”, should be taken literally. Also spiritual science speaks in this sense of the spiritual worlds. There have always been initiates who knew how to enter these kingdoms of heaven. Every religion speaks of these kingdoms. Spiritual science is but the means of disclosing anew this fundamental truth contained in every religion: Whatever we see and perceive round about us, is but the result and the effect of what takes place in the spiritual worlds. Whatever manifests itself upon the earth, is but the development of that which works and lives in the spiritual worlds.
Source: Rudolf Steiner – GA 100 – Theosophy and Rosicrucianism: Lecture I – Kassel, 16th June 1907