2 Thessalonians 3:6 KJV
Now we command you, brethren, in the name of our Lord Jesus Christ, that ye withdraw yourselves from every brother that walketh disorderly, and not after the tradition which he received of us.
Sometimes Paul exhorted and urged believers to do a thing. Sometimes he besought them. In this case Paul used the strongest term; he commanded them to withdraw from other Christians who would not follow after and live according to the traditions Paul had given them.
Paul later says that in doing so we are not to think of them as enemies, for they are not, yet we still mark them and have no fellowship with them.
Christianity in our day is becoming so feeble minded that the very idea of being commanded to do a thing is offensive to us. To be commanded to withdraw from someone and not to fellowship with them would be considered unspeakably harsh. What I see is the importance Paul places on this practice. It's not a suggestion or even a request. This is so important to the advancement of the Gospel that he issues a command. It is important because it is necessary to prevent the decline of Christian faith and practice into anything a person might want to call Christianity. We have no right, no authority to create our own traditions and culture around our faith. We are duty bound, obligated yes, even commanded to carry on our faith exactly as we have received it.