A fully realized being, Ramakrishna preached Advaita in his own words, but he would also relate stories of Rama,Ravana, Krishna and so on.
What were the scriptures he read?
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Sri Ramakrishna might not have known much of scriptures. His knowledge of scriptures must have been gathered from what he had heard from various monks or from others who were having knowledge of scriptures.
Quoting from "A Short Life of Sri Ramakrishna by Swami Tejas Ananda":
Gadadhar soon found a new source of pleasure in the company of wandering monks who used to stay for a day or two in the rest-house built by the neighbouring Laha family for wayfarers. One day Chandra was startled to find her dear boy appear before her with his whole body smeared with ashes and with pieces of cloth put on like a wandering holy man. Association with these itinerant monks and listening to their readings from the scriptures inclined the naturally emotional mind of the boy more and more to meditation and kindled in him the latent spirit of dispassion for all worldly concerns. Gadadhar was now nine, and it was time to invest him with the holy thread. A curious incident happened in this connection.