Initiation is the highest stage in an esoteric training concerning which it is possible to give some indications in a book intended for the genuine public. Whatever lives beyond forms a subject difficult to understand, yet the way to it can be found by all who have passed through preparation, enlightenment, and initiation as far as the lesser mysteries.
The knowledge and proficiency conferred by initiation cannot be obtained in any other manner, except in some far distant future, after many incarnations, by quite different means and in quite a different form. The initiate of today undergoes experiences which would otherwise come to him much later, under quite different circumstances.
Source: Rudolf Steiner – GA 10 –Knowledge of the Higher Worlds – II -The Stages of initiation