1 John 4:3

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.

(v3, NIV)

These spirits who are not from God do not even acknowledge Jesus. In fact, this is the spirit of the antichrist. John is referring again to the antichrist who he spoke about back in chapter two. The antichrist is clearly not someone you want to be listening to.

What does it mean to acknowledge or to not acknowledge Jesus? In ESV and NASB, the word used is "confess" instead of "acknowledge". To acknowledge Jesus or confess Jesus is to testify to what he truly is. We acknowledge what he is and what he said. Jesus is the son of God and he is Lord. He is our savior and our redeemer. To accept some of what Jesus is and said, yet deny other parts would be the same as someone who does not acknowledge or confess Jesus entirely. You cannot pick and choose what you believe about Jesus and claim to follow him. Instead, you are only partly following Jesus, thus following someone who is not Jesus. Those who do such things do not confess or acknowledge Jesus.

Let's say for example that I had a friend named Harry who was a nice guy (whatever that means). Let's also pretend that I thought Harry was a big jerk and I acted that way. Assuming he's a jerk will change the way I act around him or talk about him. This may affect our friendship and he may not want to continue being my friend. I don't think a situation like that could work and we could not become really good friends unless it was some sort of fake or illusion of something that it's not. Alternatively, I could have a friend named Nale who is in fact a big jerk. But for some reason, I may treat him like he's a real nice guy. His actions may betray my portrayal of him and I may end up hurt. Since he's a jerk, he may learn this fact and take advantage of me. He may pretend to be a nice guy, but that's only an illusion masking who he really is. A close friendship cannot be struck under the facade of something he is not. Those were somewhat poor examples, but you cannot accept Jesus as something he's not. You cannot claim to truly be in fellowship with him if you do not accept him for what he is. You cannot pick and choose the kind of Jesus you follow. To do such a thing would be not following Jesus at all. Acknowledge and confess Jesus.

Don't get all flustered and worried that you have something wrong. By that I mean, if you are sincerely seeking and following Jesus, I don't think you need to worry if you're mistaken about a minor principle that even true followers of Christ debate over. I've heard something along the lines like this: "On the major's, conviction. On the minors, tolerance." . There are certain things that we can disagree about yet still be followers of the Lord Jesus Christ. However, there are other things that there is no question about. In addition, if you are wrong, seek to find out what is the truth. If you think you're right, continue to seek to find out what is the truth. You may be deceived. If you are in the truth, God will continue to bless you when you run and follow hard after him.

This antichrist is not some things to beware of in the near or distant future. It's not like some slow moving iceberg in the distance. Though, a slow moving iceberg is what took down the Titanic, it can be seen from far away and is fairly slow moving. The antichrist is already in the world. They heard about it, but they should know that the antichrist is here now.