(Srila Prabhupada preaching in London, 1969—click to enlarge)
Prabhupāda: Yeṣāṁ anta-gataṁ pāpaṁ (BG 7.28). One who has finished sinful activities, he can be engaged in God consciousness. One who is engaged in sinful activities, he cannot. "Devil citing scripture." A devil cannot cite scripture. Angel can cite scripture. And according to our ācārya, Sanātana Gosvāmī, he says that "Don't hear scriptures from the devil." Avaiṣṇava-mukhodgīrṇaṁ pūtaṁ hari-kathāmṛtaṁ śravaṇaṁ naiva kartavyam (Padma Purāṇa). "If a devil's preaching about God, don't hear."
If you say, "God is pure, so let me hear about God. It doesn't matter whether he is devil or angel. It doesn't matter," No! The argument is sarpocchiṣṭaṁ payo yathā. Just like milk is very nourishing food, but if it is touched by the lips, by the tongue, of a serpent, it becomes poison. Sarpocchiṣṭaṁ payo yathā. You cannot drink the milk which is touched by the serpent's tongue.
So we should receive knowledge of God, message of God, from a person who is not a devil. Then it will be effective. Otherwise, it will act as poison. Caitanya Mahāprabhu's principle is, āpani ācari prabhu jīveri śikṣāya. He first of all behaves himself, and then he preaches. So if the preacher is sinful, how he can deliver other sinful men by his so-called preaching? That is not possible.
He must be pure, he must be sinless. Then his preaching will be effective. But in all condition, if we, with faith and love, we chant the holy name of God, we shall become gradually purified. There is no, I mean to say, cause of anxiety. Anyone who will chant this holy name, he'll be purified.
(Srila Prabhupada Lecture, London, August 13, 1972)