Unless one is situated as pure Vaisnava in his dealing, in his behavior, inside and outside, he should not become a preacher because it will not be effective; neither one should hear from such person. But people in general, they cannot understand, but those who are preaching, they must be very sincere, the same way, rupa-raghunatha pade, haibe akuti. They should read the literatures, the instruction, just like Upadesamrta, The Nectar of Instruction. We should follow, strictly follow. Then prthivim sa sisyat [Upadesamrta 1]. Then you’ll be able to preach and make disciples all over the world. This is the injunction. That is called gosvami. Don’t be cheap preacher, cheap guru. No. That is not good.

Srila Prabhupada; lecture on Srimad Bhagavatam 7.9.28 – Mayapur, March 6, 1976