Separation between Material and Spirit
God Himself, holy spirits, angels, devils, and Satan are real in the Bible. These spiritual entities are sometimes visible to men.
But there are no scenes depicted in the Gospels that Christ Jesus called upon spirits of dead men from Heaven or the hell.
So, there must be a simple rule in the spiritual domain that men cannot invoke spirits of dead men. But it does not mean that there are no spiritual entities of dead men.
Especially, the Gospels imply that Christ Jesus that appeared before His followers after His physical death on the cross was not a spiritual entity but a living man.
This universe was created in a manner so that every material and substance in this world follow the certain laws the science has made clear. There is clear distinction and separation between the material world and the spiritual world.
However, God Himself, holy spirits, angels, devils, and Satan are real in the Bible. These spiritual entities are sometimes visible to men.
Indeed if God should allow these spiritual entities to appear before men, they can, since God is the creator of the material universe and spiritual universe.
And, Christ Jesus said not to be deceived by ill spirits, because there is real risk for men to be deceived by ill spirits.
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Mar 7:18 And he saith unto them, Are ye so without understanding also? Do ye not perceive, that whatsoever thing from without entereth into the man, it cannot defile him;