karma brahmodbhavam viddhi
tasmat sarva-gatam brahma
nityam yajne pratisthitam
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.
Commentary by Srila Prabhupada
Yajnartha-karma, or the necessity of work for the satisfaction of Krishna only, is more expressly stated in this verse. If we have to work for the satisfaction of the yajna-purusha, Vishnu, then we must find out the direction of work in Brahman, or the transcendental Vedas. The Vedas are therefore codes of working directions. Anything performed without the direction of the Vedas is called vikarma, or unauthorized or sinful work. Therefore, one should always take direction from the Vedas to be saved from the reaction of work. As one has to work in ordinary life by the direction of the state, one similarly has to work under direction of the supreme state of the Lord. Such directions in the Vedas are directly manifested from the breathing of the Supreme Personality of Godhead. It is said, asya mahato bhutasya nisvasitam etad yad rg-vedo yajur-vedah samavedo ’tharvangirasah. “The four Vedas—namely the Rig Veda, Yajur Veda, Sama Veda, and Atharva Veda—are all emanations from the breathing of the great Personality of Godhead.” (Brihad-aranyaka Upanishad 4.5.11) The Lord, being omnipotent, can speak by breathing air, for as it is confirmed in the Brahma-samhita, the Lord has the omnipotence to perform through each of His senses the actions of all other senses. In other words, the Lord can speak through His breathing, and He can impregnate by His eyes. In fact, it is said that He glanced over material nature and thus fathered all living entities. After creating or impregnating the conditioned souls into the womb of material nature, He gave His directions in the Vedic wisdom as to how such conditioned souls can return home, back to Godhead. We should always remember that the conditioned souls in material nature are all eager for material enjoyment. But the Vedic directions are so made that one can satisfy one’s perverted desires, then return to Godhead, having finished his so-called enjoyment. It is a chance for the conditioned souls to attain liberation; therefore the conditioned souls must try to follow the process of yajna by becoming Krishna conscious. Even those who have not followed the Vedic injunctions may adopt the principles of Krishna consciousness, and that will take the place of performance of Vedic yajnas, or karmas.
Commentary by Sri Visvanatha Cakravarti Thakur
The cause of this action is the Veda (brahma). The actions related to the yajna arise from hearing the statements of the rules enunciated in the Vedas. The cause of the Vedas is brahman (aksaram), as the Vedas arise from the Lord. The sruti says:
asya mahato bhutasya nihsvasitam etad rg-vedo yajur-vedah sama-vedo’ thangirasah
The Rg, Yajur, Sama and Atharva Vedas are the breathing of the Supreme Lord. Brhad Aranyaka Upanisad 2.4.10
Therefore the all pervading (sarva gatam) Lord is present in the yajna. This means that by performance of the yajna one attains the Lord. Though, by cause and effect, from food to the Lord, many items have been mentioned, among them all, the yajna is mentioned by the scriptures as obligatory.
This process is mentioned in the smrti as well.
agnau prastahutih samyag adityam upatisthate adityaj jayate vrstir vrster annam tatah prajah
The offering to the fire goes to the sun. From the sun arise rain, food and bodies of the living entities. Manu Smrti 3.76