सर्वभूतेषु येनैकं भावमव्ययमीक्षते।अविभक्तं विभक्तेषु तज्ज्ञानं विद्धि सात्त्विकम्॥
sarva-bhūteṣu yenaikaṃ bhāvam avyayam īkṣateavibhaktaṃ vibhakteṣu taj jñānaṃ viddhi sāttvikam
Word by word
The verse says
The knowledge by which you see one unchanging being in everything that lives, undivided among the divided: know that knowledge as clear.
In plain words
The best kind of knowing, defined in one line.
Seeing one thing in all of them. Undivided, among things that plainly are divided.
Notice he does not say the divisions are false. People really are separate; you can count them. The claim is that something undivided is present in the counting.
Where you've felt this · in community
Your old teacher taught five-year-olds for twenty-six years. Ask her whether children have changed and she says no. The clothes change. The toys change. The words they use change every few years. What a frightened child needs on the first morning has not moved at all.
Going deeper
Seeing one thing in many without denying the many is the balance this verse strikes, and both halves are load-bearing.
It would be easy to read this as a claim that differences are illusions. The Sanskrit does not say that. It says undivided among the divided. The division is granted. Something undivided is also there.
That matters practically, because the flattening version does real damage. Telling someone that their particular situation is an illusion is not wisdom, it is a way of not listening. The clear knowledge described here would make you more attentive to a particular person, not less.
And the test of whether you have it is behavioural rather than mystical. It shows up as the ability to recognise a state you have been in yourself when it appears in someone whose life looks nothing like yours.