For the purpose of this post, I consider “sectarianism” the practice of separating from other believers (that is, people that we consider to be brothers and sisters in Christ) because of differences in beliefs other than differences concerning the gospel.
Our call to “make disciples” is to help one another follow Jesus Christ by living as citizens of the kingdom of God.
If a group (or individual) limits discipleship to their (or his/her) own group, background, denomination, tradition, organization, or even local church, then that group (or person) is not making disciples of Jesus Christ. Instead, that group is making converts to their (his/her) own group, background, denomination, etc. No group, background, denomination, etc. is equal to the kingdom of God.
Thus, sectarianism annuls discipleship.