The Belgic Confession · Article 28
Every One Is Bound to Join Himself to the True Church
We believe, since this holy congregation is an assemblage of those who are saved, and out of it there is no salvation, that no person of whatsoever state or condition he may be, ought to withdraw himself, to live in a separate state from it; but that all men are in duty bound to join and unite themselves with it; maintaining the unity of the Church; submitting themselves to the doctrine and discipline thereof; bowing their necks under the yoke of Jesus Christ; and as mutual members of the same body, serving to the edification of the brethren, according to the talents God has given them. And that this may be better observed, it is the duty of all believers, according to the Word of God, to separate themselves from those who do not belong to the Church, and to join themselves to this congregation, wheresoever God hath established it, even though the magistrates and edicts of princes be against it; yea, though they should suffer death or bodily punishment.
Therefore all those who separate themselves from the same, or do not join themselves to it, act contrary to the ordinance of God.