Shankaracharya, or Adi Shankara, was a Hindu scholar who is credited as the principal consolidator of Advaita Vedanta, one of the classical spiritual paths of India.  ‘Advaita’ means ‘not two,’ and refers to the understanding that the soul or ‘atman’ is not separate from the ultimate Reality.  Little is known for certain about Adi Shankara; various sources have placed him anywhere from approximately 500 BCE to 900 CE, but he is usually thought to have lived in the first half of the 8th century CE.  The verse quoted here gives a taste of his recognition of Unity.

Mind, nor intellect, nor ego, feeling;
Sky nor earth nor metals am I.
I am He, I am He, Blessed spirit, I am He!
No birth, no death, no caste have I;
Father, mother, have I none.
I am He, I am He, Blessed spirit, I am He!
Beyond the flights of fancy, formless am I,
Permeating the limbs of all life;
Bondage I do not fear; I am free, ever free.
I am He, I am He, Blessed spirit, I am He!