Our church has been recently going through the end of Mark 9. Jesus teaches us to receive His people, whatever their social standing may be (Mark 9:37). We are not stop others from serving Him even though their context is different from our own (9:39–41), and we greatly sin if we reject other believers and thereby cause them to sin, that is, to discourage them from following Him (Mark 9:42; cf. 9:38–41).
A question I came to ask myself is this: does my church provoke believers to sin every time we choose not to join them in some type of ministry endeavor? In giving in answer, it was helpful for me to remember that noninvolvement with other Christians is not the same thing as discrediting them as illegitimate, and the choice for noninvolvement may be based on many factors.
In seeking whether or not we should involve our church with another church, a primary question is this: what does the other church believe about the cardinal doctrines of the Christian faith? For example, is Jesus God? Is Scripture inerrant? Did Jesus die for our sins? If a church cannot properly articulate the essentials of the faith, then such a church may be no church at all or a church with aberrant beliefs at best.
A second question is this: what does the other church believe about certain distinctives of the faith that my church is convinced to be true? For me, example questions would be, are they Baptists? Dispensational? Complementarian? Hold to the regulative principle in worship? Distinctives obviously vary from one church to the next.
Answering these questions helps one to find out to what degree a church is truly for another church or not. Agreement on cardinal doctrines is essential. As to doctrinal distinctives, less and less agreement diminishes the possibility of cooperating in some way, and it is important for each church to realize that disagreement does not necessarily mean the other church is intentionally disobedient to the Word of God. We are forced to choose either to limit our beliefs for the sake of cooperation or to limit our cooperation for the sake of greater specificity in our beliefs. It is up to each church to decide which of those options they want to live out.
Practically, what would one do with other churches? As for conventions, denominations, and associations, a church must responsibly mind such organizations if they are to participate therein, meaning energy and effort must be expended outside of the local church, and nobly so, because it means participating in the larger body of Christ. A church may have to choose, though, if the benefits of the organization outweigh the potential frustrations that will come. As to other means of participation, churches may hold periodic events together if they are close enough to do so. Churches could informally or formally plant churches together or support other ministries by pooling funds and resources. By doing so, they do more together than one church alone.