Deceiver! Antichrist!

2 John 1:7-13
Many deceivers, who do not acknowledge Jesus Christ as coming in the flesh, have gone out into the world.
Anyone who runs ahead and does not continue in the teaching of Christ does not have God; whoever continues in the teaching has both the Father and the Son.
Christ's teaching is the test by which to judge visiting teachers. Those who 'go beyond' it cannot be from God.
John is combating(打擊) the same sort of trouble here as in his first letter. From the start of the Christian mission there had been travelling evangelists and teachers, usually responsible to one of the apostles, who were given hospitality by the local churches.
The time has come to tighten up. Those whose teaching contradicts the fundamental truth about Jesus Christ should not be made welcome.
John looks forward to a visit. There is so much more to be said than he can put in writing. The 'children of your dear sister' are presumable members of John's own church.
Why?
Jesus Christ as coming in the flesh is very important, because Jesus shade his blood, he suffered as our's and Jesus resurrection from the dead. All of this are based on Jesus came to world in flesh.
If he who does not acknowledge Jesus Chris as coming in the flesh, his teaching are different from us, so he is not welcome to us.
How?
Knowing the truth so we can protect us from deceiving and keep the faith in truth.