What does John teach about combatting heresy in 1 John?

It seems clear that John wrote his first epistle specifically to counter various forms of Gnostic heresy which had infiltrated the church. Gnostics believed that the spirit was good, but matter was evil, and denied the Incarnation of Christ as God made flesh. More extreme forms of Gnosticism such as Docetism suggested that Jesus was a phantom and only appeared to have a body. John emphasises in his letter the historical coming of Christ: 'which we looked upon and touched with our hands' (1 Jn 1:2); 'Jesus Christ has come in the flesh' (1 Jn 4:2).

Similarly to his gospel, John writes about polar divisions of light and darkness, truth and error, children of God and children of the devil in his letter. The false teachers seemed to have claimed a higher revelation and spirituality through their 'superior' understanding, and John is keen to stress that all men are spiritually blind and dependent on God's revelation of Himself to them in Christ. Either a person rejects Christ, and thus is in spiritual darkness, or believes in His Name. Anyone who believes in Him is in the light and there is no sense of a hierarchy of spiritual experience. Instead, John emphasises the benefits that all people receive when they believe in Jesus: they have eternal life and become children of God, with His Spirit's anointing: 'He has given us of His Spirit' (1 Jn 4:13).

As Stott points out, 1 John is marked with certainty. Believers can be certain of having eternal life, by testing themselves on whether they believe in the truth of Christ and whether they love one another. Ultimately the assurance of our salvation rests in the fact that Jesus Christ is the propitiation for our sins, which John mentions twice in his letter. God took the initiative to offer His Son as a sacrifice for our sins, and the Son willingly offered Himself: 'we have an advocate... He is the propitiation for our sins' (1 Jn 2:1-2).

David Jackman suggests that the false prophets had separated themselves from the main body of believers on their claim to a special 'anointing' of the Holy Spirit, by which they had been given true knowledge of God. John therefore emphasises and defines what a true knowledge of God really is, and the point he makes repeatedly is that true knowledge of God is shown in loving your brothers. The false teachers did not regard their unwillingness to value and love other Christians as sin; John shows them how they have completely missed the point of God's commandment: 'that we believe in the name of His Son Jesus Christ and love one another, just as He has commanded us' (1 Jn 3:23). The singular form of 'commandment' is used, and yet two things are included: believing in Christ and loving one another. This shows just how inseparable these aspects are from true faith. 'Anyone who does not love does not know God, because God is love.' (1 Jn 4:8)

John finishes his letter with a brief summary of all the assurances of the believer. Repeating the phrase 'we know', he emphasises that 'you may know that you have eternal life' (1 Jn 5:13), 'we know that he hears us in whatever we ask' (1 Jn 5:15), 'We know that everyone who has been born of God does not keep on sinning' (1 Jn 5:18) and 'We know that we are from God' (1 Jn 5:19). This culminates in his great statement: 'we know that the Son of God has come and has given us understanding, so that we may know him who is true; and we are in him who is true, in his Son Jesus Christ. He is the true God and eternal life.' (1 Jn 5:20) For John, combatting heresy is about rejoicing in the truth: Christians have a unique privileged status before God in Christ, and a unique privileged certainty that they have salvation through His blood ('your sins are forgiven for his name’s sake', 1 Jn 2:12). As long as we consciously seek to obey His commands, we have no reason to doubt His love towards us and the reality of our salvation: 'if we walk in the light... we have fellowship with one another, and the blood of Jesus His Son cleanses us from all sin.' (1 Jn 1:7)