All of religion is contained in the Vedanta

Swami Vivekananda writes  :

Dear Alasinga,
Now I will tell you my discovery. All of religion is contained in the Vedanta, that is, in the three stages of the Vedanta philosophy, the Dvaita, Vishishtâdvaita and Advaita; one comes after the other. These are the three stages of spiritual growth in man. Each one is necessary. This is the essential of religion: the Vedanta, applied to the various ethnic customs and creeds of India, is Hinduism. The first stage, i.e. Dvaita, applied to the ideas of the ethnic groups of Europe, is Christianity; as applied to the Semitic groups, Mohammedanism. The Advaita, as applied in its Yoga-perception form, is Buddhism etc.

Now by religion is meant the Vedanta; the applications must vary according to the different needs, surroundings, and other circumstances of different nations. You will find that although the philosophy is the same, the Shâktas, Shaivas, etc. apply it each to their own special cult and forms. Now, in your journal write article after article on these three systems, showing their harmony as one following after the other, and at the same time keeping off the ceremonial forms altogether. That is, preach the philosophy, the spiritual part, and let people suit it to their own forms. I wish to write a book on this subject, therefore I wanted the three Bhashyas; but only one volume of the Ramanuja (Bhashya) has reached me as yet.

Ever yours with blessings,
Vivekenanda

PS. Nobody will come to help you if you put yourself forward as a leader. . . . Kill self first if you want to succeed. (V, 81 - 82) (From the letter to Alasinga from U.S.A. dated 6th May, 1895.)