Not only is the Supreme Personality of Godhead present as the Supersoul of all living entities; at the same time, He pervades everything in the entire creation. He exists in all circumstances and at all times. He exists in the heart of Lord Brahmā and also in the cores of the hearts of the hogs, dogs, trees, plants and so on. He is present everywhere. He is present not only in the heart of the living entity, but also in material things, even in the atoms, protons and electrons being explored by material scientists.
The Lord is present in three features—as Brahman, Paramātmā and Bhagavān. Because He is present everywhere, He is described as sarvaṁ khalv idaṁ brahma. Viṣṇu exists beyond Brahman. Bhagavad-gītā confirms that Kṛṣṇa, by His Brahman feature, is all-pervading (mayā tatam idaṁ sarvam (BG 9.4)), but Brahman depends upon Kṛṣṇa (brahmaṇo hi pratiṣṭhāham (BG 14.27)). Without Kṛṣṇa, there could be no existence of Brahman or Paramātmā. Therefore, Bhagavān, the Supreme Personality of Godhead, is the ultimate realization of the Absolute Truth. Although He is present as the Paramātmā in the core of everyone's heart, He is nonetheless one, either as an individual or as the all-pervading Brahman.
The supreme cause is Kṛṣṇa, and devotees who have surrendered to the Supreme Personality of Godhead can realize Him and His presence within the universe and within the atom (aṇḍāntara-stha-paramāṇu-cayāntara-stham (BS 5.35)). This realization is possible only for devotees who have fully surrendered unto the lotus feet of the Lord; for others it is not possible. This is confirmed by the Lord Himself in Bhagavad-gītā (BG 7.14):