B.G 4.35

यज्ज्ञात्वा न पुनर्मोहमेवं यास्यसि पाण्डव। येन भूतान्यशेषेण द्रक्ष्यस्यात्मन्यथो मयि ॥३५॥

yajjñātvā na punarmohamevaṁ yāsyasi pāṇḍava। yena bhūtānyaśeṣeṇa drakṣyasyātmanyatho mayi ॥35॥

[यत् ज्ञात्वा न पुनः मोहम् एवं यास्यसि पाण्डव। येन भूतानि अशेषेण द्रक्ष्यसि आत्मनि अथो मयि॥
yat = that which; jñātvā = having known / understood; na = not; punaḥ = again; moham = delusion; evaṁ = thus; yāsyasi = you will go; pāṇḍava = Arjuna;। yena = by which; bhūtāni = living beings; aśeṣeṇa = without exception; drakṣyasi = you will see; ātmani = in the 'ātman'; atho = therefore; mayi = in Me;]

Having gained that knowledge, O Arjuna, you will never get deluded again. Possessed with such a knowledge, you will see all living beings residing in Me, without exceptions.

Gīta Tātparya 4.35

'ātmani mayi' - in all pervading Me. The word 'atho' indicates, because of the lord's all pervasiveness (all living beings residing in the lord).

आत्मनि व्याप्ते मयि। अथो तस्माद् व्याप्तत्वादेव ॥३५॥

ātmani vyāpte mayi। atho tasmād vyāptatvādeva ॥35॥

[ātmani = the word 'ātmani' means; vyāpte = being pervaded; mayi = in Me;। atho = the word 'atho' indicates; tasmād = therefore; vyāptatvādeva = i.e. because of His pervasiveness alone; ॥35॥]

The word 'ātmani mayi' means residing in all pervading Me. The word 'atho' which means therefore indicates, because of His all pervasiveness alone (you will see all living beings residing in Me).