BG Chap 17 - Performing sacrifice, charity and penance by uttering Oṁ tat sat
Sacrifice, charity and austerity should always be performed along with uttering the names of Viṣṇu. Those following the rules should also understand the spiritual goal of all actions. Baladeva Vidyābhūṣaṇa says that those in the mode of goodness should perform these by uttering Oṁ. Viśvanātha Cakravartī says that among those in goodness, the seekers of Brahman should perform these by uttering Oṁ. The three words Oṁ tat sat are remembered by the wise to be the names of Viṣṇu. These names are representative only, since there are many more. Among these words, Oṁ is known in the śrutis as the name of Brahman. The word tat is known to designate the cause of the universe, and is also known to indicate the obliterator of that which is not tat – the material world. Sat is defined as that which exists before everything else. All these words indicate Brahman. The brāhmaṇas, Vedas and sacrifices were manifested by Brahmā in the form of the three names Oṁ tat sat, indicating Viṣṇu. Thus these names are very powerful. If these names are used along with sacrifice and other acts, these names will compensate for any fault in the performance of the various parts of the sacrifice. By uttering these three sounds, one is assured of getting the results. Thus the three higher castes perform sacrifices, austerities and charity after chanting Oṁ.
Uttering tat, persons aspiring for liberation performed sacrifice, austerity and charity without seeking results. The word tat is very powerful because it produces a desire for liberation by uttering it with absence of desire.
The word sat is used in relation to Brahman as in sad eva saumya (that Brahman is eternal) and also in relation to the seekers of Brahman as in satāṁ prasaṅgāt (from association with the knowers of Brahman). Thus both the Absolute Truth, who is the objective of devotional sacrifice, and the performer of such sacrifice are indicated by the word sat. The Supreme Lord indicated by the word sat exists in all excellent things. Thus the word sat is used in performing all auspicious activities, whether material or spiritual, such as upanayana and marriage. What is permanent in sacrifice, austerity and charity is called sat. Sat is situated in those works by being the goal of those works. Any work done for the sake of the Lord such as building or cleaning His temple, are also called sat. Thus all the works of sacrifice, penance and charity, to be true to the absolute nature, should be performed to please the Supreme Person.
To summarize Oṁ represents Viṣṇu. Tat is chanted to achieve liberation. All activities performed for the Lord’s pleasure are sat, eternal. Thus these words ultimately refer to the process of devotional service. One must remember these three names. This should be considered an injunction because it is necessary to utter these names during sacrifice, charity and other actions in order to remove all obstacles for attaining the final result.