The Belgic Confession of Faith
Article 28: The Obligations of Church Members
We believe that, since this holy assembly and congregation is the gathering of those who are saved and there is no salvation apart from it, believers ought not to withdraw from it, content to be by themselves, regardless of their status or condition.
But all people are obliged to join and unite with it, keeping the unity of the church by submitting to its instruction and discipline, by bending their necks under the yoke of Jesus Christ, and by serving to build up one another, according to the gifts God has given them as members of each other in the same body.
And to preserve this unity more effectively, it is the duty of all believers, according to God’s Word, to distinguish themselves from those who do not belong to the church, in order to join this assembly wherever God has established it, even if civil authorities and royal decrees forbid and death and physical punishment result.
And so, all who withdraw from the church or do not join it act contrary to God's ordinance.