THE MYSTICAL UTTERENCE, OM TAT  SAT, PURIFIES ALL ACTIONS

om tatsaditi nirdesho brahmanas trividhah smritah
    braahmanaastena vedaashcha yajnaashcha vihitaah puraa  // 17.23 //

`OM TAT SAT' - this is considered to be the threefold  symbol of Brahman. By this were ordained of old the Brahmanas, the Vedas and  the sacrifices.

Om  Tat Sat is a sentence of three words, each denoting one of the aspects of the  Reality. 
•`OM' represents the Transcendental and the Pure Self, Absolute and Unborn which is the substratum upon which the projections of the body, mind and intellect are maintained. 
•`Tat' indicates the Eternal Goal, the Changeless and the Ever-Perfect, that from which everything has come, in which everything exists and into which everything merges back in the end. 
•`Sat' means existence, the principle functioning through all things.

Invoking  this sentence purifies all our activities and motives behind them. When invoked  in ritualism it removes all defects in the worship, it being the name for  Brahman.

The  Eternal Reality indicated in the triple nomenclature of Brahman `Om Tat Sat' is  the source from which all castes, the Vedas and Sacrifices commenced. All super  impositions arise from, exist on and disappear into that which is their own  substratum, the Brahman.

tasmaadomityudaahritya yajnadaanatapahkriyaah
    pravartante vidhaanoktaah satatam brahmavaadinaam  // 17.24 //

Therefore,  with the utterance of OM the acts of gift,  sacrifice and austerity enjoined in the scriptures are always begun by the  expounders of Brahman.

tadityanabhisandhaaya phalam yajnatapah kriyaah
    daanakriyaashcha vividhaah kriyante  mokshakaangkshibhih // 17.25 //

And  with the utterance of the word `Tat' the acts of sacrifice and austerity and  the various acts of giving are performed by the seekers of salvation without  aiming at any reward.

sadbhaave saadhubhaave cha sadityetatprayujyate
    prashaste karmani tathaa sacchabdah paartha yujyate  // 17.26 //

The  word `Sat' is used in the sense of Reality and of goodness and so also, O Partha,  it is used in the sense of an auspicious act.

The  context in which the terms - Om, Tat, and Sat  - have to be used is explained here. Sri Krishna says that when the acts of  sacrifice, gift and austerity are undertaken by the seekers the term OM is to be uttered to cherish in the mind the divine  awareness of the supremacy of the Infinite. This adds purpose and meaning to  all our acts of sacrifice, charity and austerity by freeing the mind from the  egocentric attachments.

Liberation  means liberation of our personality from its physical, emotional and  intellectual attachments. Once liberated the human personality realizes the  Divinity in itself. One who endeavors to liberate himself must perform all his  activities in such a way that the causes that create the attachments -vasanas-  are completely eliminated.

To  work in the field of Yagna or Tapas or Dana by understanding the meaning of the  term `Tat' i.e. the universal oneness of the spiritual truth is to work with no  ego and consequent freedom from attachments. With the utterance of `Tat' alone  which indicates the universal truth of oneness of all living creatures all acts  of sacrifice, penance and gifts are undertaken by the seekers of liberation  without expecting any reward. The word `Sat' means both reality and goodness.  It is also used for all praiseworthy actions.

In  our daily contact with the world of objects we more often believe that this  world, as physically observed, is the reality although it is only relatively  real in comparison to the unchanging substratum, the `Sat', the Absolute  Reality.