Many people had told me that Ramakrishna Paramahamsa's wife Devi Sarada was an avatar of Goddess Jagadamba/Durga. Is this true?
Yes, this is truth that Devi Sarada was a form of Goddess Durga. Sri Ramakrishna Paramhansa had worshipped Devi Sarada because he thought that his mother Kali or Jagadamba had came in the form of his wife Sarada Devi. I also believe that Devi Sarada was an avatar of Goddess Durga.
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