A brahmana who is born out of the head of the Lord has as his business to preach the transcendental Vedic sounds, or sabda-brahma. Because the brahmana is the head, he has to preach the transcendental sound, and he also has to eat on behalf of the Supreme Lord. According to Vedic injunctions, when a brahmana eats it is to be understood that the Personality of Godhead is eating through him. It is not, however, that the brahmana should simply eat on behalf of the Lord and not preach the message of the Bhagavad-gita to the world. Actually, one who preaches the message of the Gita is very dear to Krsna, as is confirmed in the Gita itself. Such a preacher is factually a brahmana.

From The Nectar of Devotion ch. 2
by Srila Prabhupada