1. The Church is God's "called out people." You can't join it; God adds you to it. Membership is limited to a very clearly defined group: "whosoever" (or, in other words, anyone and everyone who wants to be a member can be a member).
The status of your membership is never up for preachers, priests, or Popes to determine; once you establish your own connection with God, no power under Heaven can shatter it.
The Church exists for one reason: to bring people closer to God and closer to each other. The concept of The Church is radical and revolutionary — primarily because it empowers you (and only you!) to explore a personal relationship with the Divine.
2. The organizations most people call "churches" are really just clubs. Clubs — communities of people, organized around a common interest — are human institutions … and churches, as they exist today, function very much like clubs.
There are conditions, defined by the members, which potential members must meet in order to join. Once a member, there are other conditions which govern the status of your membership; in addition, there are individuals empowered to judge the extent to which you meet those conditions.
Follow the rules and fulfill the conditions, and you're "in." Break the rules or fall short of the conditions, and you're "out."
3. There's a difference in being a member of a club .. and a member of The Church. Churches — just clubs, really — may, if they are lucky, count among their members a few people who are, in fact, members of The Church. But no church — which is, really, just a club — should ever be confused with the objective, vital reality that is The Church.
4. The spiritual legitimacy of any club varies directly with the degree to which that club looks and acts like The Church. There's no harm in joining any club. If you like what the organization is up to, by all means — join it. And if your primary reason for attending a church is to take advantage of the social, financial, or service-oriented opportunities found there — to use it in the same way many people use country clubs or fraternities — well, there are plenty of clubs from which to choose.
But if your reasons for joining the club are spiritual — that is, if you're joining in hopes of being more fully immersed in and connected to The Church — the question you must ask is, "To what degree does this club look and act like The Church?"