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).
What is your religion? That would be a cult. Any religion that denies christ is a cult.
I'm Roman Catholic. We usually capitalize the "c" in Christ, to indicate that we recognize that He is the Lord our God & Savior. If I'm not mistaken, that is common practice in protestant denominations also.
There are Christian cults that don't deny Christ. Most of the ancient heresies that pop up again and again are connected with whether Christ had one body or two or one spirit or two. A lot of mischief arose over the roles of the Persons of the Trinity in relation to each other.
This is a nice page at ChurchHistory101.com that briefly describes the major ancient heresies. They make a short mention of Marcion. He became a teacher and produced the first major version of the bible that was widely available. It consisted only of Paul's epistles and the book of Luke. And no mentions of hell at all. It proved so popular at the time that it actually led to the church establishment of the era to adopt the canon of scripture we still use today following the list first circulated by Athanasius, a hero of the fight against Arianism heresy. (Arianism insisted that Christ was not eternal but was created by the Father.)
Anyway, this is how we got the bibles we have because the bishops finally realized that someone had to produce an official recognized canon of books. Lots of heretical books were floating around at the time, phony religious texts like the Gospel of Judas and hundreds of others.
Gary is a real expert at this stuff, has really impressive resources. It often surprises me that people don't find this stuff as fascinating as I do but I guess it's pretty ancient news so it's considered dull. Yet, these same heresies pop up century after century in one form or another.