If mediums who claim to ‘summon’ Bodhisattvas and Buddhas can really do so, then these enlightened ones should start the most excellent of Dharma classes, and be able to answer challenging technical questions about passages in the sutras. However, few, if any, will ever ask them, because most go to mediums to solve worldly problems instead of seeking greater wisdom.
Yet, if the enlightened ones really possess mediums, they would be able to skilfully lead devotees to learn the Dharma, even if gradually. But no, this never happens. At least, not in the cases I hear. On the contrary, there is often advice offered that is misaligned or only partially aligned with the Dharma.
There is a kind of ghost who likes to possess mediums, to speak through them. As they can read minds, it is not difficult for them to know the answers the devotees expect to hear, and to answer accordingly. This is one of the reasons why true Buddhists don’t go to ghosts (and gods) for refuge or advice, while the Buddhas are perfect in compassion and wisdom, whom many ghosts and gods take refuge in instead, listening to their advice.
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Are Manifestations & Mediums Of Bodhisattvas & Buddhas Around?
http://thedailyenlightenment.com/2014/02/are-there-mediums-manifestations-of-buddhas-bodhisattvas-around