Warning against False Teachers: 1 John 4:1-6. 
John warns believers against the false teachers. There are many false teachers in the world and they cannot be trusted.  They should be tested by the teaching of Christ and the apostles (Acts 17:11).  We must test them whether they are from God or Satan (4:1, 6).  John mentioned two ways to know whether a teaching is from God or not. First, those who deny that  "Jesus Christ has come in the flesh" are not from God (1 John 4:3).  True believers trust in the incarnation of Christ (John 1:14).  Do they confess that Jesus Christ has come in the flesh from God to be our savior?  False teachers deny the incarnation of Jesus Christ.  The second way to know if a teaching is from God is to test whether it is consistent with the teaching of the apostles. False teachers do not have the Holy Spirit and they are controlled by antichrist.  The more popular a teaching is does not make it more correct.  False teachers do not recognize Jesus as the only way of salvation. They do not give glory to God but seek human approval. The Holy Spirit always glorifies Christ.  Accepting Christ as a prophet or great teacher or humanitarian will not save anybody.  People have to believe in Christ to be saved. The true believers are able to understand truth from error because they have the Holy Spirit. The Spirit who lives in them is greater than the spirit who is in the world.  The false teachers are inspired by the spirit of the world. They seek the approval of men rather than of God. John concludes that those who follow the Spirit of Truth will accept the teaching of the apostles as the Word of God and reject the errors (1 John 4:6). The false teachings have just enough truth to impress people but it is dangerous and deceptive. Test every teaching carefully against God's Word (Acts 17:11).

Christians Love One Another: 1 John 4:7-21.  
John again exhorts believers to love one another in order to show that they are different from the false teachers. Love is the proof that our faith is real (1 John 4:7).  Love is of God and everyone who loves is born of God. The proof of God’s great love is clearly seen on the cross. God so loved the world that He gave His only Son to die on the cross for our sins (John 3:16). He paid the penalty of our sin to redeem us from the bondage of Satan. He loved us when we were enemies of God (Rom 5:8). Only Christ can take away our sins and give us eternal life. We love him because he first loved us (4:19). This love should motivate us to love one another. When we love each other, God abides in us and His love is perfected in us (1 John 4:12).  It is the proof that we have the Holy Spirit and we are in Christ.   God has sent His Son to be the Savior of the world. Because Christ is our Savior and lives in us, we can love each other. Our obedience to God is not a fearful striving to please God but a thankful response to his undeserving grace toward us. There is no fear in love. This gives us confidence to stand before the judgment seat of Christ without any fear of punishment (2:28).  John argues that it is impossible to love God and at the same time hate brothers (4:20). This love is more than an emotion and it should be demonstrated in action. The gospel is the good news of God's unfailing love and grace provided in Christ Jesus.  The gospel is indeed the source of great comfort and hope.