Seeking unity
Unity with other existing churches
In addition to continually examining our own church in light of God’s Word, we must, over time, also look around at other churches and assess whether they can be recognized as churches that are built on the same foundation. If this is the case after investigation and discussions, we should strive for mutual recognition and unification. In doing so, we serve Christ’s work of gathering His church. This work is not static in the sense that everything remains the same all the time, but dynamic. Christ can disrupt existing church institutions and create new ones. For example you can look to the situation in the nineteenth century with the Secession and Doleantie in the Netherlands. Christ’s work is only complete at His return at the Last Day.
A church federation for churches within one country, and foreign sister church relationships for churches abroad
If there are nationally more than one true congregation, we must form a church federation as an implementation of God’s command, as articulated in Article 28 BC. The essential aspect is to demonstrate that we are the body of Christ, deeply and spiritually connected to one another through the Spirit. We should collectively bend our necks under the yoke of Christ and serve each other in brotherly unity as members of the same body. This yoke of Christ demands obedience to His teachings, the edification of His church, and the interaction within His body through the communion of saints. This includes keeping watch over one another and, if necessary, correcting one another through objections. This also involves establishing and maintaining a church federation. 
Any form of fellowship with other Christians outside the church federation is not in line with the unity God requires. The church federation itself is part of the yoke of Christ. The communion of saints equals the one holy catholic Christian church as we confess in the Apostles’ Creed. This communion cannot be found outside that one church.
Therefore, we must separate from a church that has ceased to be the true church. This is a divine command that we must not oppose but obey, even if the government opposes it with force or if it results in breaking family or friendship ties. The Lord Jesus is very clear about this. We are not allowed to leave the true church either, as this also contradicts God’s command.
As a consequence, we must also not have fellowship with others without the God-ordained unity. This occurs by establishing an ecclesiastical fellowship with another church in the same area where you both do not fully submit to the yoke of Christ. How can this be reconciled with Ephesians 4:1-16, which contains such an important description of the unity of the church?
In addition to establishing and maintaining a church federation within our own country, we must also strive for sister church relationships with true churches abroad, to the extent possible based on accessibility, language, and culture. This also requires a continual attitude of seeking and assessing.
How to interact with other Christians?
Can we as a church not collaborate at all with non-true churches or church federations? On a personal level, we can come into contact with well-meaning Christians. We can speak with them and try to invite them to attend the church. What about cooperation? A distinction must always be made between matters related to synusia (society) and koinonia (communion of saints sharing the Holy Supper). We must realize that other Christians are not automatically our brothers, but we should address them and, when possible, encourage them to obediently follow Christ.
Inviting speakers who do not belong to the true church for church meetings such as conferences for office-bearers is generally undesirable. Office bearers should be reliable and not instructed or educated by unreliable speakers. The same applies to teachers from outside the church in Reformed schools.
Collaboration in the areas of evangelism and mission is excluded as long as there is no union as true churches. This also applies to the participation in consultative bodies such as councils of churches. The membership of such councils obscures the borders of the church and leads to confusion about the true church. This could lead to disobedience to the one Head, Christ, and promote an unscriptural pluriformity of the church. The Lord asks for sincerity and transparency instead of cover-up in these crucial matters.
(to be continued)