What should I say to a family member who claims to be a Christian but refuses to join a church?

Questions like this are often pastoral questions that need to be explored in context. I would need to know what the family member believes and what your relationship with the family member is.

I think a good place to start would be Hebrews 10:25: “Neglect not the gathering of yourselves together as is the habit of some.” Ask your family member, “Why does the Bible warn us against not gathering together?”

Maybe this person does gather with the church but just hasn’t joined. In that case, you could look at 1 Corinthians 12 where Paul talks about being members of one another. That might not strictly address a membership role, but it does talk about the church's organic unity and connectedness.

You could also look at passages in Scripture about discipline. How does a church discipline someone who is not part of the church? Discipline is crucial to the life of the Christian community, as we see at many points in Scripture.

As the opportunity arises, I would try to engage this family member in examining certain passages of Scripture that relate to this issue. It is also important to determine why they have not joined a church—were they offended or abused in some way? It is a good opportunity to explore the origins of this problem and be in a better position to address it.