1 John 4:1

"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." NIV translation

Satan is the father of lies and these days he has sent out many liars who are trying to deceive people. John tells us to test the spirits so that we can tell whether they are from God or Satan.

1 John 4:2 & 3a

"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, but every spirit that does not acknowledge Jesus is not from God." NIV translation

It is a simple test to know who sent someone as we see. Anyone that does not accept that Jesus is the Son of God is not sent by God. There are many that want to say things like "Jesus was a great prophet" or "Jesus was a great man" but deny that He is and was God in the flesh. These people are deceived and not sent by God.

1 John 4:3b

"This is the spirit of the antichrist, which you have heard is coming and even now is already in the world." NIV translation

There are two different meanings to antichrist. The first describes a particular person that is to come and we see this in Revelation 13. The second meaning is what we see in this verse and it describe those who are against (anti) the fact that Jesus was God in the flesh. John says that this antichrist spirit is already in the world and can be most clearly seen demonstrated in Islam.