Sanatana Gosvami has forbidden strictly, that avaisnava-mukhodgirnam putam-hari-kathamrtam, sravanam na kartavyam [Padma Purana]: “Don’t hear from the professional men who are not situated in the Vaisnava behavior.” One must be…, behavior in Vaisnava. Sadacara-sampannah. Srila Visvanatha Cakravarti Thakura has warned that “Don’t mix it with Vaisnava who is not well trained up, well behaved.” You can offer him respect. A Vaisnava offers respect even to the ant. That is another thing. But he has warned, “Don’t mix with them, these professional, so-called Vaisnava, sahajiyas.” This is warned. Avaisnava-mukhodgirnam puta-hari-kathamrtam, sravanam na kartavyam: “You should not hear.” “Eh? What is the wrong there? The Krsna-katha is there.” No, Krsna-katha is there just like milk is there, but if it is touched by the lips of a serpent, it is no more to be taken. It has become poison.

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