Perhaps no other Mystic influenced all three Abrahamic faiths as did Plotinus [203-270 CE]. He famously joined Gordian’s march on Persia to get to India, failed, and returned to Antioch. In 3rd Century Bharath, the understanding of Shūnyam was still not entirely dead.
The earliest expansion of ‘Not-Two’ in the literature is as: ‘One without a Second’.
The notion of ‘The One’[Sanskrit: Ekam Sat] is universal and grades and variants of it can be found in every literate tradition from Plotinus to that of that of the Abrahamic Faiths which later gets conceptualized and reified into a ‘Monotheism'[See the Posts].
The Neo-Platonic ‘One’ of Plotinus [sourced in Plato’s Parmenides] was the dominant European Mystical Tradition for a millennia. It influenced everyone from Thomas Aquinas to Meister Eckhart.
Reality, wrote Plotinus was: ‘A Nameless Unity, indescribable, undefinable.. never known measure, stands outside number..is under no limit of any kind..is Everything and Nothing..’.