How can we know if someone has a demon in them?

I know of two ways to have this knowledge. The perception of demons comes as a result of the gifts of the Spirit.

First Corinthians 12:7-11 lists these gifts:
But the manifestation (gifts) of the Spirit is given to every man to profit withal. For to one is given by the Spirit the word of wisdom; to another the word of knowledge by the same Spirit; To another faith by the same Spirit; to another the gifts of healing by the same Spirit;

To another the working of miracles; to another prophecy; to another discerning of spirits; to another divers kinds of tongues; to another the interpretation of tongues: But all these worketh that one and the selfsame Spirit, dividing to every man severally as he will.
(1 Cor. 12:7-11 KJV)
Two of these nine gifts enable us to be aware of the presence of a demon in another person. These gifts are a word of knowledge and discerning of spirits.

Word of Knowledge
The word of knowledge operates when the Holy Spirit speaks to us (makes us aware) of something that has happened, or is happening in another person's life.

This could include the presence of a spirit or demon inside of them. The Spirit of God would simply tell you without you seeing the spirit.

Discerning of Spirits
The discerning of spirits gives you the ability to see into the spirit realm. You may actually see the spirit being. I have operated in the word of knowledge in this way and cast a devil out.

I also have friends that have operated in discerning of spirits, saw the demon, and then cast him out.

If you don't have the Holy Spirit's revelation of a demon through one of these two gifts, you should not try to cast one out of a person. Their problem may be mental or physical instead of demonic.

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