The question is how Vedic Hindus worship their supreme formless God Brahman.
1) By Karma Yoga:
By doing YAGNA, Vedic Hindus worship their supreme formless God Brahman.
Reciting Vedic scriptures, doing Vedic Yagnas and Vedic Homas are the ways of worshiping God Brahman.
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Majority of people can worship Brahman only by Karma yoga.
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2) By Jnana Yoga:
Paroksha & Aparoksh jnan:
Jnan a is of two kinds - Paroksha - i.e. which you learn from books or Acharyas, and Aparoksh - i.e. which you learn by your own Anubhuti or realization.
Once the spiritual thirst is created, the seeker naturally turns towards spiritual books or spiritual teachers for light on the path. He acquires theoretical knowledge, Paroksha Jnana. This, as we noticed, is the first step, and a necessary step. The next step in spiritual life, and the most essential step, is practice.
Practice ultimately leads to Aparoksha Anubhuti or the supreme Direct Experience. Indirect knowledge becomes direct experience. All that is learnt and understood about Brahman is more than verified by one’s own experience. That brings down the final curtain on the drama of life. But between the initial stepping on to the spiritual path and the final experience of realising Brahman, there are innumerable facts of spiritual life which the Sadhak verifies for himself personally. And with each verification his conviction in the truth of what he learnt grows.
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Only very very few can become brahma jnanis like Suka Brahmam
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3) Bhagavad Gita: Chapter 3, Verse 14
"annad bhavanti bhutani
parjanyad anna-sambhavah
yajnad bhavati parjanyo
yajnah karma-samudbhavah"
Synonyms:
annat--from grains; bhavanti--grow; bhutani--the material bodies; parjanyat--from rains; anna--of food grains; sambhavah--are made possible; yajnat--from the performance of sacrifice; bhavati--becomes possible; parjanyah--rains; yajnah--performance of yajna; karma--prescribed duties; samudbhavah--born of.
Translation:
All living bodies subsist on food grains, which are produced from rains. Rains are produced by performance of yajna [sacrifice], and yajna is born of prescribed duties.
4) Bhagavad Gita: Chapter 3, Verse 15:
"karma brahmodbhavam viddhi
brahmaksara-samudbhavam
tasmat sarva-gatam brahma
nityam yajne pratisthitam"
Synonyms:
karma--work; brahma--Vedas; udbhavam--produced from; viddhi--one should know; brahma--the Vedas; aksara--the Supreme Brahman (Personality of Godhead); samudbhavam--directly manifested; tasmat--therefore; sarva-gatam--all-pervading; brahma--transcendence; nityam--eternally; yajne--in sacrifice; pratisthitam--situated.
Translation:
Regulated activities are prescribed in the Vedas, and the Vedas are directly manifested from the Supreme Personality of Godhead. Consequently the all-pervading Transcendence is eternally situated in acts of sacrifice.
5) Madhvacarya's Commentary on Gita Verses (3, 14-15):
Actions are born from the Vedas because the Vedas prescribe yagna or worship and appeasement and the performance of yagna is done by actions. The Vedas arose out of the breath of the Supreme Brahman. This is noted by the word brahmaksara which means the supreme immutable Vedas which was the medium by which the knowledge of the ultimate reality was revealed.
Otherwise how could the Supreme Brahman which is inconceivable, without a beginning and an end and complete within Himself ever been known. The word aksara in brahmaksara means the ultimate reality denoting the Supreme Lord. When a primary meaning has been identified it is not proper to accept a secondary meaning as is often the case due to forgotten traditions. Insentient objects cannot manifest by themselves they must be created. The magnitude of actions in creation, the unlimited resources of elements and the awesome manifestation of time indicate inconceivable powers.
Since the material worlds are but reflections of the spiritual worlds all actions in material existence emanate outwards with the energy of the Supreme Lord as the source. This does not directly include the actions exhibited by sentient beings using free will only the underlying energy that empowers them.
The word aksarani refers to the eternality of the Supreme Lord. The eternal Vedas have Him for their source. Because He is eternal the Vedas are eternal as well. There would be a flaw in anyone thinking that the Vedas were created because they could not have been so without prior knowledge. They are eternally manifest and the Vedic scriptures reveal that they were exhaled by the breath of the Supreme Being. Exhaling is a natural phenomena without any effort required indicating that the Vedas manifested from the spiritual worlds without any conscious effort, this is the inference meant here.
In view of being eternal the statements of being created have to be understood from the point of view of manifesting within material existence or perception of such not that it was actually created for the first time. An example of this is the treatise Shatapatha Brahmana which is said to have been created by Yagnavalkya. How is it possible for any part of the eternal Vedic scriptures to be created by anyone? Yagnavalkya blessed by the Supreme achieved a high level of attainment and received it within his mind from the spiritual plane and transcribed it onto palm leaves for posterity.
A statement clarified after deliberation is superior to one spoken without contemplation. The Vedic scriptures are the source of origin, thus the statement in Brahma Sutras I.I.III that the Vedas are the evidence that Brahma is the creator. In the previous verse I.I.II the word janmadyasya is used meaning from whom were created is the evidence which can be quoted. These verses do not suggest that the Vedas were born from Brahman nor do they infer the Brahman was the cause of the Vedas.
The creating of the material worlds by the Brahman from the five basic elements given to him of water, earth, air, fire and ether does not preclude the creation of the Vedas nor would it be correct to assume that the Vedas were created this way. The creation of a most wonderful world is in no way superior to the eternal emanations of the Vedas. This also does not lead to the conclusion that the Brahman is omniscient even though the manifestation of the Vedas is omniscient. Therefore the eternal Vedas coming from the spiritual worlds are themselves the proof and the word aksara confirms this.
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So Brahman is revealed through yagna and Brahman is revealed through Vedic scriptures and the Vedas eternally establish yagna throughout the material creation. From knowledge of the Vedas and performance of yagna living entities prosper and flourish. This is the eternal cycle.
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Sources:
Answer got from my father, aged 84, a vedic pandit in Krishna Yajur Veda.