Nonduality, or Advaita says that all existence is one unified whole. Advaita has it’s roots in the Upanishads, Hindu texts dating from around the seventh or sixth centuries B.C.E.
The great early teacher of Avaita, Shankara lived between 788 and 820 C.E.
One of it’s greatest modern teachers was Ramana Maharshi (30 December 1879 – 14 April 1950).
Sri Nisargadatta Maharaj (17 April 1897 – 8 September 1981) was another great modern teacher.
There is also a history of nondual thought in Western culture.
Today there are many modern nondual teachers (some may reject the term teacher but I use it here for brevity) including Rupert Spira, Lisa Cairns, Mooji and Jeff Foster.