When Paul came to Thessalonica in Acts 17, he began preaching in the synagogue with some success (v. 1-4). However, many of the Jews became upset and caused an uproar (v. 5). They searched for Paul and Silas, the ones who had "turned the world upside down" with their teaching (v. 6). When they could not find them, they brought Jason and some of the other brethren before the city authorities (v. 6-7). What charge did they bring against Jason? He "welcomed them" (v. 7).
This reminds me of a passage in 2 John. John wrote, "If anyone comes to you and does not bring this teaching, do not receive him into your house, and do not give him a greeting; for the one who gives him a greeting participates in his evil deeds" (2 Jn. 10-11). John is teaching that when one supports, welcomes, or receives one who does not bring the "teaching of Christ" (2 Jn. 9), he becomes a partner in evil. The Jews in Thessalonica saw Paul and Silas as evil men. In their minds, Jason, because he welcomed them, was just as deserving of punishment and they were. This is the principle of 2 John 10-11. But the punishment for receiving one that John talked about will not come from civil authorities, but from "Him who is able to destroy both soul and body in hell" (Mt. 10:28).
It is interesting that some of the enemies of the cross of Christ were able to grasp this principle of fellowship while some of our own brethren cannot. Some Christians and local churches today welcome false teachers, support compromisers, and receive those who are unwilling to bring "the whole counsel of God" (Acts 20:28). When they do this, they become a partner in wickedness and stand just as condemned as the one they receive.
First and foremost, "our fellowship is with the Father, and with His Son Jesus Christ" (1 Jn. 1:3). Let us not forfeit our relationship with God for any man, no matter how popular, friendly, knowledgeable, zealous, etc., he is.
I just recently read an article, 2 John 9-11 and Fellowship by Tim Haile. If you are interested in reading more on the topic of fellowship, I would recommend that article.