First, the universal church should not be set against the local church as though they were exclusive categories, or as though one could nullify the need for the other.
Second, interpreting the doctrine of the universal church to permit spiritual individualism leads to an insufficient view for explaining the family and one another passages of the New Testament, or for sustaining healthy spiritual life.
Two illustrations: The Human family
Every person ever born is a member of what we call the human family. That is, they are instantly a part of a large, innumerable collection of other human beings.
But, being born into a human family does not make a person a member of the family called "Jones" that lives on Main Street. Not everyone born into the human family can make a claim on the love, resources, and care of the Jones family. Imagine you're in the mall and a little fella you've never seen before in your life comes running up to you with a Nintendo Wii and an arm full of video games saying, "Oooh! Oooh! Oooh! Mommy/Daddy please buy this for me. Please can I have it?! It's only $800."
Obviously, the little fella is confused. You don't accept this kind of begging from your own children. He's a human sure enough, a part of that larger family. But he's not an Anyabwile or a Jones or a Smith (as common as those names are). You'd politely direct the tike to find "his real parents." You might go so far as to help him look for or walk him to the nearest help desk or security counter. But you wouldn't regard him as a member of your family and entitled to your family's ongoing resources and love.
For that to happen, something else must occur. There's a switch that must be flipped. There must be a common ownership or commitment, where members of the Jones family give themselves to the child and the child gives itself to the care of the Jones family as a participating member. Whether this "switch" gets flipped by a natural birth or by adoption, there must be the recognition of mutual belonging.
Now such mutual belonging in a particular family doesn't mean the child is no longer a part of the human family. He certainly is. The categories are not in contradiction. The Jones family is a particular of the general.
Moreover, belonging to the family Jones is what ensures that the child will remain a part of the human family. Let me try to explain that with another family illustration.
Imagine you're not at the mall, but taking out the garbage. You approach the can and there you see an infant wrapped in a blanket. Someone has left them there to be discovered. What would you do?
Well, if we're a part of the human family we'd recognize the humanity before us. We would likely temporarily take the child into our care and notify the proper authorities. That's the only humane thing to do. We recognize that unless that child receives the care of a surrogate family or some agency acting in that capacity, he will not survive. He will perish alongside that garbage can. What's necessary for their survival is participation in a particular family where mutual belonging is experienced. We would not wish to leave the child abandoned or see them orphaned. We would recognize that for a member of the human family lack of belonging to a particular family is unhealthy or abnormal or less than ideal .
Mutual belonging. That, I think, is the key to understanding church membership and why it's important. It's also the key to understanding why arguing "I'm a member of the universal church and therefore don't have to join a local church" is finally to wrongly put in opposition two things that belong together and to deny the kind of ongoing care necessary to the spiritual life of Christ.
At bottom mutual belonging in a family (or, local church membership if you will) rests on three things:
1. Recognition of a person's new humanity (being a part of the universal church, or the "human family" to use the language of the illustration) by a credible testimony of faith and conversion;
2. Recognition by the family (the local church) of a desire, responsibility, and commitment to care for an individual as one of its own in a continuing relationship; and
3. Recognition by the individual of a desire, responsibility, and commitment to care for and participate in the life of the entire family (the local church).
When these things are present, we can say the "switch" of mutual belonging has been flipped. Membership in the Jones family or the Second Baptist family has just occurred. We give expression to our universal spiritual life by our belonging to a particular spiritual family.
Making Mutual Belonging Clear to All
Now, one last thing to consider. The strength of this mutual belonging depends in part on how clearly that mutuality is established. In other words, we may have informal members of our families (like "uncle" Bernie, who really isn't a blood relative but a friend of the family) or more formal/clearly identified members of the family. Though we love "uncle Bernie" and may treat him like family, the designation "like family" indicates a difference in degree between true members and informal members.
The clarity and strength of this mutual belonging is bolstered when a local church has a clear process for taking in and seeing out members. The process can take any number of forms. Form isn't necessarily the critical thing. The critical thing is how explicit the process is in aiding the three recognitions we mentioned earlier: credible profession of faith; commitment of the church to the individual; and commitment of the individual to the church.
Being unclear at any of those points will have weakening effects on the local church and perhaps the individual. This is why claimants who say "we can do these same things with our friends down the street and not join the church" almost always drift toward spiritual decay rather than spiritual vibrancy.
But being careful and clear, helps each member of the family to grow in its relationships with the other members and with Christ Jesus. Just as we need the Jones, Smith, Anyabwile, Carter, and Johnson families to be strong... we need the First Baptist, Third Presbyterian, Fourth Apostolic Succession, and even the Ever-Abounding-Higher-Life-Now families to be strong. Of course, there is more to being strong than clarifying membership... but strong local church membership practices help.