"The authority of the Vedas is unchallengeable and stands without any question of doubt. And whatever is stated in the Vedas must be accepted completely, otherwise one challenges the authority of the Vedas.
"The conchshell and cow dung are bone and stool of two living beings. But because they have been recommended by the Vedas as pure, people accept them as such because of the authority of the Vedas."
The idea is that one cannot set his imperfect reason above the authority of the Vedas. The orders of the Vedas must be obeyed as they stand, without any mundane reasoning. The so-called followers of the Vedic injunctions make their own interpretations of the Vedic injunctions, and thus they establish different parties and sects of the Vedic religion. Lord Buddha directly denied the authority of the Vedas, and he established his own religion. Only for this reason, the Buddhist religion was not accepted by the strict followers of the Vedas. But those who are so-called followers of the Vedas are more harmful than the Buddhists. The Buddhists have the courage to deny the Vedas directly, but the so-called followers of the Vedas have no courage to deny the Vedas, although indirectly they disobey all the injunctions of the Vedas. Lord Caitanya condemned this.
The examples given by the Lord of the conchshell and the cow dung are very much appropriate in this connection. If one argues that since cow dung is pure, the stool of a learned brāhmaṇa is still more pure, his argument will not be accepted. Cow dung is accepted, and the stool of a highly posted brāhmaṇa is rejected.