• Hare Krsna,
        Blaspheming devotees is considered a great offense because firstly, devotees are dear to the Lord. They are serving him with their total devotion and are unable to tolerate if anyone commits an offense against Krsna. Similarly, the Lord like a parent is unable to tolerate any offenses against his child who is showering pure love, and in this way it's an exchange of pure love between Krsna and his devotee. Therefore, Krsna is unable to tolerate any offenses against his devotees who have dedicated their lives. Committing vaisnava offense is like a mad elephant spoiling all the fruits of devotional service, and the result of our Bhakti. Even Krsna cannot forgive this, and the only way is to apologize to the devotee offended.
        The second point is that useless quarrel and argument can cause anger which can also result in lust/greed consequently causing devotees to fall down.

        Hope I answered your question,
        yrs, Tamal Krsna
        • What about if you tell the truth about someone doing something and it offends that person or someone else, is that considered vaisnava aparadha?
          • Jerry, 

            Quarrels (read: debates) about nyaya - meaning Justice, and vaisesika - meaning Materialism were a commonplace occurrence in ancient India, where an intellectual debated with other intellectuals on Justice, Liberty, Business, Science, Astronomy, Language - any topic under the sun. But it was deemed dangerous for the ruling elite clergy (Swamis/Gurus/Pandits) as this encouraged and inculcated a scientific dialogue on material things and made people question the status quo (read: scriptural inerrancy). Hence, a lot of rules and regulations were put into place to limit progressive thought and keep power concentrated to a few, basically put the fear of God in people. 

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