In verse 10 John sets out one practical application of how to defend the truth: If anyone comes to you and does not bring this teaching, do not receive him into your house. Hospitality for traveling teachers was common in the culture (cf. Luke 9:1-6; 10:1-12). The prohibition here is not to turn away the ignorant; it does not mean that believers may not invite unbelieverseven those who belong to a cult or false religioninto their midst. That would make giving the truth to them difficult, if not impossible. The point is that believers are not to welcome and provide care for traveling false teachers, who seek to stay in their homes, thereby giving the appearance of affirming what they teach and lending them credibility
Johns use of the conjunction ei (if) with an indicative verb indicates a condition that is likely true. Apparently, the lady to whom he wrote had for whatever reason, in the name of Christian fellowship, already welcomed false teachers into her home. It was just such compassionate, well-meaning people that the false teachers sought out (cf. 2 Tim. 3:6); since churches were supposed to be protected by elders who were skilled teachers of the Word (1 Tim. 3:2; Titus 1:9), they should have been less susceptible to the lies propagated by the deceivers. Having established themselves in homes, the false teachers hoped eventually to worm their way into the churches. It is much the same today, as false teaching insidiously invades Christian homes through television, radio, the Internet, and literature.
So threatening are these emissaries of Satan that Jo[h]n went on to forbid even giving them a greeting; for the one who gives him a greeting participates in his evil deeds. Irenaeus relates that the church father Polycarp, when asked by the notorious heretic Marcion, Do you know me? replied, I do know youthe firstborn of Satan (Against Heresies, 3.3.4). John himself once encountered Cerinthus (another notorious heretic) in a public bathhouse in Ephesus. Instead of greeting him, however, John turned and fled, exclaiming to those with him, Let us fly, lest even the bath-house fall down, because Cerinthus, the enemy of the truth, is within (Irenaeus, Against Heresies, 3.3.4).
" The Amish and Mennonites never knock on my door. "
Years ago, I had Jehovah's Witnesses come to my house, while my wife & I were carrying groceries in, while it was raining. They followed me up & down the stairs, back & forth from the front door to the car. I tried to be polite & cordial. But they would not leave. I told them we already belonged to a church. When they asked which one, I told them we were Jewish.
They looked at me like I had told them I was satan, and left abruptly.
I did this because of a story a State Trooper friend told me. he did the same thing, except he told them he was an atheist. The next day, got a call from Post to get home quick. When he got home, there were about 50 in his front yard, hold some kind of deal to save him. he had to threaten to take them all to jail to get them to leave, lol.
Years ago, I had Jehovah's Witnesses come to my house, while my wife & I were carrying groceries in, while it was raining.
When I was stationed in England I had them walk right in the side door without knocking or announcing their presence. Needless to say they didn't get a rousing welcome from me.