The first thing is being divisive, making divisions and causes of falling contrary to the apostles' teaching (Rom. 16:17). In Romans 14 Paul said that we must receive all kinds of genuine believers, whether they keep certain days or consider every day alike, whether they eat herbs or eat meat (vv. 1-6). Then, in chapter fifteen Paul said that we must receive genuine believers according to Christ (vv. 5-7). In these portions of the Word, Paul was very broadminded, very generous, and, in a sense, very liberal. But at the end of chapter sixteen Paul said, "Now I beg you, brothers, keep a watchful eye on those who make divisions and causes of falling contrary to the teaching which you have learned, and turn away from them." These two things—the making of divisions and causes of falling—have been and still are going on among us. Thus, we need to keep a watchful eye on those who make divisions and causes of falling contrary to what we have learned of the apostles, and we must turn away from them.
In dealing with the divisive ones, we must also take Paul's word in Titus 3:10, "A factious man after the first and second admonition refuse." A factious person is a divisive and sectarian person. According to Paul's word in Romans 16:17 and in Titus 3:10, after the first and second admonitions, we must reject such persons and turn away from them. To do this is to quarantine such persons. To quarantine a person does not mean that we hate that person. If one member in a family became ill with a contagious disease, the rest of the family would need to quarantine him for the safety of the whole family. Otherwise, the entire family would be affected. Likewise, to practice Paul's word in Romans 16:17 and in Titus 3:10 is to quarantine the divisive members in the Body of Christ. This is not my teaching; it is my presentation of the holy Word to you.
The second thing for which a believer can be kept away from the fellowship of the church is being heretical, denying that Christ came in the flesh (2 John 7-11). This implies the denying of Christ's redemptive work, which was accomplished by Christ in His flesh (Rom. 8:3; Col. 1:22). Teaching heresy is not merely a matter of teaching wrongly or differently. In 1 Timothy 1:3 Paul said that he had urged Timothy to remain in Ephesus in order that he might charge certain ones not to teach differently. However, in 2 John 7-11, John was not referring to those who taught differently, but to those who taught heretically, denying that Christ came in the flesh, that is, denying Christ's person in His humanity, and denying His redemptive work. They do not believe that Christ is both God and man who came in the flesh to accomplish redemption for us, the sinners. The apostle John said that if such a one comes to us, we should not receive him into our home, and we should not even greet him. Then John said that if we greet him, saying to him, "Rejoice," we will share in his sin. This is not my word, but the word of the apostle John.
Thus far, among us there is no one who is teaching heretically. However, there are some who are making divisions and causes of falling.