From the One came the many, and the many are the One.
In the Gospel of Thomas Jesus suggests that everything consists of a single and indivisible reality of which we are all parts. This Monist perspective suggests that we live with the mistaken perception that things exists separately from each other. This idea of the inherent Oneness of all things has infiltrated human history. Even Einstein suggested that the idea that we are separate from everything else is an optical delusion of consciousness. The earliest reference to the concept is found is in the 'Upanishads' written by seers who had retired to the forest to live contemplative lives. These writings frequently reference the idea that the self and the divine are one and the same thing, and recommend the obliteration of all distinctions and multiplicities.
We should not necessarily associate Oneness with divinity. The Neo-Platonist Plotinus suggested that the Oneness brought itself into being and is indicative of a neutral absolute. Spinoza on the other hand suggests that the Oneness is God and only God. The academic Walter Stace calls the Oneness an undifferentiated unity and suggests that the experience of it occurs beyond our intellect and our sensory faculties.