The Masnawi Of Maulana Rumi
This is pantheism, which is shirk. The trinity is too a kind of this shirk in which three gods are considered as one. God in Quran calls it Shrik. The same idea has been extended in pantheism. Academically this notion came from Plotinus, a Greek philosopher, and penetrated into other religious philosophies.
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I would also like to add that non-dualism and its various forms predate Plotinus and Neo-platonism. This concept can be found in Hinduism, Bhuddism, Taoism, Judaism, Christianity and Islam. The denial of higher metaphysical principles is only a recent phenomenon. It’s a facet of modernism which emphasizes becoming over being, actions over contemplation, appearances over reality. If you want to know more about this, you can read The Crisis of the Modern World by Rene Guenon and watch this short 37 min lecture on this book https://youtu.be/ffa3xSwV6Ms