Of all sinful activities an offense to a Vaishnava is the most severe.

Of all sinful activities, an offense to a pure devotee, or Vaisnava, is the most severe. An offense at the lotus feet of a Vaisnava is so disastrous that Sri Chaitanya Mahaprabhu has compared it to a mad elephant that enters a garden and causes great havoc by uprooting many plants and trees. If one is an offender at the lotus feet of a brahmana or Vaisnava, his offenses uproot all his auspicious activities. One should therefore very carefully guard against committing vaisnava-aparadha, or offenses at the lotus feet of a Vaisnava. Here the Lord clearly says that although Hiranyakasipu had received benedictions from Lord Brahma, these benedictions would be null and void as soon as he committed an offense at the lotus feet of Prahlada Maharaja, his own son. A Vaisnava like Prahlada Maharaja is described herein as nirvaira, having no enemies. Elsewhere in Srimad-Bhagavatam (3.25.21) it is said, ajata-satravah santah sadhavah sadhubhusanah: a devotee has no enemies, he is peaceful, he abides by the scriptures, and all his characteristics are sublime. A devotee does not create enmity with anyone, but if someone becomes his enemy, that person will be vanquished by the Supreme Personality of Godhead, despite whatever benedictions he may have received from other sources. Hiranyakasipu was certainly enjoying the fruitful results of his austerities, but here the Lord says that as soon as he committed an offense at the lotus feet of Prahlada Maharaja he would be ruined. One’s longevity, opulence, beauty, education and whatever else one may possess as a result of pious activities cannot protect one if one commits an offense at the lotus feet of a Vaisnava. Despite whatever one possesses, if one offends the lotus feet of a Vaisnava he will be vanquished.

Source: A.C. Bhaktivedanta Swami Prabhupada (2014 edition), “Srimad Bhagavatam”, Seventh Canto, Chapter 04 – Text 28