"Having material desires, men worship various demigods. They have no idea how to get out of matter; they want to utilize the material world to its best capacity. For instance, in Vedic literature there are so many recommendations: if one wants to cure his disease, he worships the sun, or if a girl wants a good husband, she worships Lord Siva, or if one wants to become beautiful, he worships such and such god, or if one wants to become educated, he worships goddess Sarasvati. In this way Westerners often think that the Hindus are polytheistic, but actually this worship is not to God, but to demigods. We should not think that the demigods are God. God is one, but there are demigods who are also living entities just like us. The difference is that they have a considerable amount of power. On this earth there may be a king or a president or a dictator--these are men like us, but they have some extraordinary power, and in order to get favors from them, to take advantage of their power, we worship them in one way or another. But Bhagavad-gita condemns worship of the demigods. This verse clearly states that people worship the demigods due to kama, material lust.
....There are many rules and regulations for the worship of demigods in the scriptures, and one may question why the Vedic literatures recommended their worship. There is necessity. Those who are motivated by lust want the opportunity to love something, and the demigods are acknowledged as the officers of the Supreme Lord. The idea is that as one worships these demigods, he will gradually develop Krsna consciousness. But if one is completely atheistic and disobedient and rebellious against any authority, what hope is there? So one's obedience to a higher personality can start with the demigods.
If, however, we take directly to the worship of the Supreme Lord, worship of the demigods is not necessary. Those who worship the Supreme Lord directly show all respect to the demigods, but they do not need to worship them because they know that the supreme authority behind the demigods is the Supreme personality of Godhead, and they are engaged in worshiping Him. In any case, respect is still there. A devotee of the Lord shows respect even to an ant, what to speak of the demigods? The devotee is aware that all living entities are parts and parcels of the Supreme Lord and that they are playing different roles only."
On The Way to Krsna, Chapter 5
"....A person engaged in discharging duties in Krsna consciousness is also called a perfect yogi or a first-class mystic. But there are others also, who perform similar sacrifices in the worship of demigods, and still others who sacrifice to the Supreme Brahman, or the impersonal feature of the Supreme Lord. So there are different kinds of sacrifices in terms of different categories. Such different categories of sacrifice by different types of performers only superficially demark varieties of sacrifice. Factual sacrifice means to satisfy the Supreme Lord, Visnu, who is also known as Yajna. "
All the different varieties of sacrifice can be placed within two primary divisions: namely, sacrifice of worldly possessions and sacrifice in pursuit of transcendental knowledge. Those who are in Krsna consciousness sacrifice all material possessions for the satisfaction of the Supreme Lord, while others, who want some temporary material happiness, sacrifice their material possessions to satisfy demigods such as Indra, the sun-god, etc. And others, who are impersonalists, sacrifice their identity by merging into the existence of impersonal Brahman.
The demigods are powerful living entities appointed by the Supreme Lord for the maintenance and supervision of all material functions like the heating, watering and lighting of the universe. Those who are interested in material benefits worship the demigods by various sacrifices according to the Vedic rituals. They are called bahv-isvara-vadi, or believers in many gods. But others, who worship the impersonal feature of the Absolute Truth and regard the forms of the demigods as temporary, sacrifice their individual selves in the supreme fire and thus end their individual existences by merging into the existence of the Supreme. Such impersonalists spend their time in philosophical speculation to understand the transcendental nature of the Supreme. In other words, the fruitive workers sacrifice their material possessions for material enjoyment, whereas the impersonalist sacrifices his material designations with a view to merging into the existence of the Supreme. For the impersonalist, the fire altar of sacrifice is the Supreme Brahman, and the offering is the self being consumed by the fire of Brahman. The Krsna conscious person, like Arjuna, however, sacrifices everything for the satisfaction of Krsna, and thus all his material possessions as well as his own self--everything--is sacrificed for Krsna. Thus, he is the first-class yogi; but he does not lose his individual existence."
Bhagavad-gita 4:25 Purport
"Unintelligent men, who know Me not, think that I have assumed this form and personality. Due to their small knowledge, they do not know My higher nature, which is changeless and supreme.
Purport: Those who are worshipers of demigods have been described as less intelligent persons, and here the impersonalists are similarly described. Lord Krsna in His personal form is here speaking before Arjuna, and still, due to ignorance, impersonalists argue that the Supreme Lord ultimately has no form. Yamunacarya, a great devotee of the Lord in the disciplic succession from Ramanujacarya, has written two very appropriate verses in this connection. He says,
"My dear Lord, devotees like Vyasadeva and Narada know You to be the Personality of Godhead. By understanding different Vedic literatures, one can come to know Your characteristics, Your form and Your activities, and one can thus understand that You are the Supreme Personality of Godhead. But those who are in the modes of passion and ignorance, the demons, the nondevotees, cannot understand You. They are unable to understand You. However expert such nondevotees may be in discussing Vedanta and the Upanisads and other Vedic literatures, it is not possible for them to understand the Personality of Godhead." (Stotra-ratna 12)
In the Brahma-samhita it is stated that the Personality of Godhead cannot be understood simply by study of the Vedanta literature. Only by the mercy of the Supreme Lord can the Personality of the Supreme be known. Therefore in this verse it is clearly stated that not only are the worshipers of the demigods less intelligent, but those nondevotees who are engaged in Vedanta and speculation on Vedic literature without any tinge of true Krsna consciousness are also less intelligent, and for them it is not possible to understand God's personal nature. Persons who are under the impression that the Absolute Truth is impersonal are described as abuddhayah, which means one who does not know the ultimate feature of the Absolute Truth. In the Srimad-Bhagavatam it is stated that supreme realization begins from the impersonal Brahman and then rises to the localized Supersoul--but the ultimate word in the Absolute Truth is the Personality of Godhead. Modern impersonalists are still less intelligent, for they do not even follow their great predecessor, Sankaracarya, who has specifically stated that Krsna is the Supreme Personality of Godhead. Impersonalists, therefore, not knowing the Supreme Truth, think Krsna to be only the son of Devaki and Vasudeva, or a prince, or a powerful living entity. This is also condemned in the Bhagavad-gita (9.11). Avajananti mam mudha manusim tanum asritam: "Only the fools regard Me as an ordinary person." The fact is that no one can understand Krsna without rendering devotional service and without developing Krsna consciousness. The Gita confirms this.
Bhagavad-gita 7:24 Text and Purport
Bhaktivedanta Book Trust. HDG A.C. Bhaktivedanta Swami Srila Prabhupada.