Īśāvāsyopaniṣat 07
यस्मिन् सर्वाणि भूतानि आत्मैवाभूद् विजानतः। तत्र को मोहः कः शोकः एकत्वमनु पश्यतः ॥७॥
yasmin sarvāṇi bhūtāni ātmaivābhūd vijānataḥ। tatra ko mohaḥ kaḥ śokaḥ ekatvamanu paśyataḥ ॥7॥
He who understands the supreme lord, in whom all beings existed, in whom alone they take shelter, for such a person who perceives that one lord (principle) in all beings, where is delusion, where is grief?
Īśā.Bhāshya 07
He is the supreme lord, in whom all beings existed. The verse conveys the omnipresent attribute of the lord. A reference is given to Pippalāda vedic testimonial, where the concept is reiterated, using similar words.
In which supreme being exists all beings, he is the supreme lord. There, all beings existed even before. Thus, for one who knows that the supreme lord resides uniformly in all beings, where is delusion?
"In whom all beings exist, He is the 'atman' of all beings. Thus, whoever sees Vishnu everywhere, really knows Him. What delusion or grief can there be for him? He attains Vishnu."
- stated thus in the Pippalāda vedic testimonial.
As per the earlier explained translation, absence of grief and delusion indicates the correct understanding. By practicing (i.e. by seeing the presence of the lord in all beings), the omnipresent attribute of the lord comes to light.