“That art thou” is the pinnacle of teaching in the Chandogya Upanishad, a central text of Hinduism.
“In the beginning, there was Existence alone; that is one only, without a second.
He thought ‘Let me be many and let me grow forth.’ Out of Himself, He thus projected the universe.
He entered into every being and every thing. All that is has its self in Him alone.
He is the Self. And that, Svetaketu, That art Thou.”
They called this “existence itself” Brahman, and the individual soul they called Atman. Hence the central spiritual revelation: Atman is Brahman.
The different religions conceive of “existence itself” by different names, but they all have some form of “that art thou” moment.
I think it ultimately comes down to the individual. For some, being “children of God” is the most powerful and moving expression of our relationship with the divine.
For others “it is no longer I who live, but Christ lives in me” makes the most sense.
In fact in Christianity there are countless ways of expressing the good news that we are not separate from God.
In Buddhism we see a new approach to convey non-separation by doing away with the hallowed Hindu notion of atman. Anatman means “no atman”. There is no self and no Brahman. All is “empty” of self, sunyata.
Yet this emptiness is itself enlightenment, another realisation that we are not separate from all that is.
And as Buddhism evolved people found new ways of expressing non-separation, focusing on Buddha-nature within ourselves and all sentient beings, focusing on Buddhas and Bodhisattvas in devotion, prayer, and oneness, with numerous esoteric and mystical practices and sects each having their own emphasis and nuance.
Whatever the teaching, all seek to bring us to a point of realising we are not and have never been separate from the power that created all existence. And further that we should therefore fear nothing, want for nothing, but instead dwell in the love, joy, and contentment of being one with the divine.