Just as with all spiritual practices associated with experiencing the dead and departed....people not only leave themselves open for being deluded.....but they invite it....and the devil loves nothing better then to send his cohorts to fulfill that desire.
Voodoo
Hindu
Islam
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You do have to be very careful, I agree. There are charlatans all about us. The Church, however, does the work for us, in determining whether one should be canonized. And it would be counter-productive of the devil, regardless, to allow the types [and number] of miracles that came about through the intercession of St. Gemma Galgani.