Paul just finished asking for prayers in the previous section (read here). In this section Paul addresses the second major issue facing the Thessalonian church (the other issue can be read about here and here).
Paul begins with a strong statement. He commands the church by the authority of Jesus to withdraw from every brother who walks disorderly (3:6). This is a strong exhortation for several reason. First, Paul evokes the authority of Jesus. Second, “Command” starts the Greek sentence, thus providing it with added emphasis. Third, the command that Paul gives is that they “withdraw.” The word means “to avoid” or “shun.” What Paul is commanding, in an emphatic way, is what we now call dis-fellowshipping (for another article on this subject click here).