John has explained that we need to abide in God and when we do we will exhibit the righteousness of Christ in our love for our fellow believers. When we live like this then we abide in God and He abides in us by the Spirit He has given us (John 14:23). As believers, we all have the Holy Spirit within us (1 Cor 3:16) but we can grieve the Holy Spirit when we don’t live rightly (Eph 4:30-32). Once again John is speaking about abiding and fellowship. When we obey God, then we live in fellowship with Him—we abide in Him and He in us. If we grieve the Holy Spirit, He doesn’t leave us, but His relationship with us cannot be considered abiding.

If we know we are abiding by the testimony of the Spirit within us then we must be certain we can discern the Holy Spirit from all other spirits. John seems to be using the term spirit quiet loosely to cover discerning between all teachings of truth or error. Any teaching which fully confesses Jesus as Christ incarnate (in flesh) is from God. Those that do not hold to the doctrine of Jesus fully God and fully man, the Christ are not to be trusted. John doesn’t speak of those who deny but those who do not confess. Some false teaching may not be fully recognised because instead of denying a doctrine, it may just not affirm it. Because we have the Holy Spirit within us who is greater than he who is in the world (the devil and his false teachers) we will be able to stand against deception.
John speaks of three groups, you—the believers, they—the false prophets, and we—the apostles. The false prophets have a message from the world which is attractive to the world who listen to them. The apostles are from God and whoever knows God listens to them and those who are not from God don’t. This is how we know the Spirit of truth from the spirit of error, the apostolic teaching of the New Testament is where we find truth and anything that doesn’t agree with it fully, is error.

It has been said that to spot a fake, you need to know the real thing intimately. There are too many ways to present something false that it is impossible to win by looking for the error. We must focus on spending time in God’s word so we know it so intimately that when something contradicts it the error is alarmingly obvious.