SB 5.20.5

pratnasya viṣṇo rūpaṁ yat
 satyasyartasya brahmaṇaḥ
amṛtasya ca mṛtyoś ca
 sūryam ātmānam īmahīti
Synonyms: 
pratnasya — of the oldest person; viṣṇoḥ — Lord Viṣṇu; rūpam — the form; yat — which; satyasya — of the Absolute Truth; ṛtasya — of dharma; brahmaṇaḥ — of the Supreme Brahman; amṛtasya — of the auspicious result; ca — and; mṛtyoḥ — of death (the inauspicious result); ca — and; sūryam — the demigod Sūrya; ātmānam — the Supersoul or origin of all souls; īmahi — we approach for shelter; iti — thus.
Translation: 
[This is the mantra by which the inhabitants of Plakṣadvīpa worship the Supreme Lord.] Let us take shelter of the sun-god, who is a reflection of Lord Viṣṇu, the all-expanding Supreme Personality of Godhead, the oldest of all persons. Viṣṇu is the only worshipable Lord. He is the Vedas, He is religion, and He is the origin of all auspicious and inauspicious results.
Purport: 

Lord Viṣṇu is even the Supreme Lord of death, as confirmed in Bhagavad-gītā (mṛtyuḥ sarva-haraś cāham). There are two kinds of activity — auspicious and inauspicious — and both are controlled by Lord Viṣṇu. Inauspicious activities are said to be behind Lord Viṣṇu, whereas auspicious activities stand before Him. The auspicious and the inauspicious exist throughout the entire world, and Lord Viṣṇu is the controller of them both.

In regard to this verse, Śrīla Madhvācārya says:

sūrya-somāgni-vārīśa-
 vidhātṛṣu yathā-kramam
plakṣādi-dvīpa-saṁsthāsu
 sthitaṁ harim upāsate

There are many lands, fields, mountains and oceans throughout the creation, and everywhere the Supreme Personality of Godhead is worshiped by His different names.

Śrīla Vīrarāghava Ācārya explains this verse of Śrīmad-Bhāgavatam as follows. The original cause of the cosmic manifestation must be the oldest person and must therefore be beyond material transformations. He is the enjoyer of all auspicious activities and is the cause of conditional life and also liberation. The demigod Sūrya, who is categorized as a very powerful jīva, or living entity, is a representation of one of the parts of His body. We are naturally subordinate to powerful living entities, and therefore we can worship the various demigods as living beings who are powerful representatives of the Supreme Personality of Godhead. Although the worship of the sun-god is recommended in this mantra, He is worshiped not as the Supreme Personality of Godhead but as His powerful representative.

In the Kaṭha Upaniṣad (1.3.1) it is said:

ṛtaṁ pibantau sukṛtasya loke
 guhāṁ praviṣṭau parame parārdhe
chāyātapau brahmavido vadanti
 pañcāgnayo ye ca tri-ṇāciketāḥ

“O Nāciketā, the expansions of Lord Viṣṇu as the tiny living entity and the Supersoul are both situated within the cave of the heart of this body. Having entered that cavity, the living entity, resting on the chief of the life airs, enjoys the results of activities, and the Supersoul, acting as witness enables him to enjoy them. Those who are well-versed in knowledge of Brahman and those householders who carefully follow the Vedic regulations say that the difference between the two is like the difference between a shadow and the sun.”

In the Śvetāśvatara Upaniṣad (6.16) it is said:

sa viśvakṛd viśvavidātmayoniḥ
 jñaḥ kālākāro guṇī sarvavid yaḥ
pradhāna-kṣetrajña-patir guṇeśaḥ
 saṁsāra-mokṣa-sthiti-bandha-hetuḥ

“The Supreme Lord, the creator of this cosmic manifestation, knows every nook and corner of His creation. Although He is the cause of creation, there is no cause for His appearance. He is fully aware of everything. He is the Supersoul, the master of all transcendental qualities, and He is the master of this cosmic manifestation in regard to bondage to the conditional state of material existence and liberation from that bondage.”

Similarly, in the Taittirīya Upaniṣad (2.8) it is said:

bhīṣāsmād vātaḥ pavate
 bhīṣodeti sūryaḥ
bhīṣāsmād agniś candraś ca
 mṛtyur dhāvati pañcamaḥ

“It is out of fear of the Supreme Brahman that the wind is blowing, out of fear of Him that the sun regularly rises and sets, and out of fear of Him that fire acts. It is only due to fear of Him that death and Indra, the King of heaven, perform their respective duties.”

As described in this chapter, the inhabitants of the five islands beginning with Plakṣadvīpa worship the sun-god, the moon-god, the fire-god, the air-god and Lord Brahmā respectively. Although they engage in the worship of these five demigods, however, they actually worship Lord Viṣṇu, the Supersoul of all living entities, as indicated in this verse by the words pratnasya viṣṇo rūpam. Viṣṇu is brahma, amṛta, mṛtyu — the Supreme Brahman and the origin of everything, auspicious and inauspicious. He is situated in the heart of everyone, including all the demigods. As stated in Bhagavad-gītā (7.20), kāmais tais tair hṛta-jñānāḥ prapadyante ’nya devatāḥ: those whose minds are distorted by material desires surrender unto the demigods. People who are almost blind because of lusty desires are recommended to worship the demigods to have their material desires fulfilled, but actually those desires are not fulfilled by the material demigods. Whatever the demigods do is done with the sanction of Lord Viṣṇu. People who are too lusty worship various demigods instead of worshiping Lord Viṣṇu, the Supersoul of all living entities, but ultimately it is Lord Viṣṇu they worship because He is the Supersoul of all demigods.