1 Dear friends, do not believe every spirit, but test the spirits to see whether they are from God, because many false prophets have gone out into the world.
In this verse, we see that spirit and prophet is used interchangeably. This context tells us that the spirit of truth in verse 6 below is a human Prophet and not a ghost or a spook!
2 This is how you can recognize the Spirit of God: Every spirit that acknowledges that Jesus Christ has come in the flesh is from God,
“The Spirit of God” is a singular being and does not refer to any person making the claim. General public is also addressed right after this title. Prophet Muhammad (peace be upon him) acknowledged that Jesus was Messiah (Christ) and came in flesh contrary to Paul who believed that Jesus was a ghost/spook.
3 but every spirit that does not acknowledge Jesus is not from God. This is the spirit of the Antichrist, which you have heard is coming and even now is already in the world.
This verse explains that Antichrist to come is already in the world. Prophet Muhammad (peace be upon him) confirmed this point and stated that Dajjal (Antichrist) is locked or chained in a cave somewhere and will come near the end of the world. This verse agrees to it exactly and is a slap on the face of those who call Islam and its last Prophet as the Antichrist.
4 You, dear children, are from God and have overcome them, because the one who is in you is greater than the one who is in the world.
The one inside the people being addressed was not a spook as explained above but refers to the one in their heart, i.e. Prophet Muhammad (peace be upon him). The people knew that he was to come and he was in their hearts.
5 They are from the world and therefore speak from the viewpoint of the world, and the world listens to them.
6 We are from God, and whoever knows God listens to us; but whoever is not from God does not listen to us. This is how we recognize the Spirit of truth and the spirit of falsehood.
Spirit of truth (paraclete) is considered as a ghost by some Christians. This verse and the explanation above refutes this myth.