Must someone be baptized before he or she can join a church? I've been told by people who know history better than I do that Christians of almost all traditions have said yes for 2000 years. It's really only today that people have thought otherwise.
The historical precedent is pretty compelling in and of itself. It is also helpful to think about what a local church is and what church membership is, which I have tried to do in two previous posts. But understanding the relationship between baptism and membership finally requires us to consider the purpose of baptism. To do that, let's start with a story. We will call this story...
Must Wearing the Team Jersey Precede Playing with the Team?
Player: "Hey coach, the team owner just hired me. I'm ready to play."
Coach: "Great, let's get your jersey on and put you out on the field."
Player: "Wait a second, I'm not comfortable wearing a jersey. I'd prefer to hold off. Maybe I'll play a few games, and then consider wearing the jersey."
Coach: "Well, no, actually, you have to wear a jersey before you can play for us. It's how everyone knows who you are playing for."
Player: "That's ridiculous. First, I admit the rule book talks about players wearing jerseys, but nowhere does it explicitly say that I HAVE to wear a jersey BEFORE the first game..."
Coach: "Ahhh, hmmm, you're right. The rule book doesn't actually say that baptism must come before membership. Maybe we should not require our team to wear their jerseys at all. Some will; some won't. Nobody will be confused by that."
Player: "You're being sacrastic."
Coach: "Yes, I am. But tenderly so. Look, the rule book says players must wear jerseys--period. It doesn't say before or after the first game. It just says they have to wear them. And the point is, you need them from the start because those jerseys are the very thing which tell people whose team you belong to. That's what this little rite is for.
Player: "Okay, fine. But I haven't got to my second point."
Player: "Second, I still think you're being a little legalistic. I mean, I'm a team member! The team owner hired me. I don't need the jerseys to prove that I'm a member. It's a done deal. So now I want to go and play, and I think I will play best wearing my old gym shorts."
Coach: "True, the owner hired you, and that's what made you a team member. I'm glad he did. But the owner ALSO wrote rule book which said that all the players have to wear uniforms. And he delegated to me the authority to make sure you wear it. So jersey up!"
Is my parable making sense? Let's start with baptism. Baptism is like a team jersey. To "put it on" is to publicly identify yourself with the Trinity. That's what Jesus means when he speaks of being baptized "into the name" of Father, Son, and Spirit. When you are baptized, you are saying, "I'm with them!" You are putting on the team jersey.
What is local church membership? At its heart, it is the same thing. It is a declaration that we belong to Christ's kingdom and to his universal church. (See discussion here.) How does a local church make that declaration? It does it through baptism (and the Lord's Supper).
So go find my own local church's directory of names. Inside you will find all the people who we have collectively taken responsibility for as members of the universal body of Christ. We have taken responsibility to declare this short list of names to be "Christ's church" whenever we administer baptism and receive the Lord's Supper.
Must baptism precede membership? Yes. I'd even say, of course! I suppose it is possible you could have an extraordinary situation where the order might get reversed by a few weeks. Getting the order right is not a matter of ontological or salvific necessity, per se. But yes, it generally must precede it, because of what these things are. Baptism (and the Lord's Supper) is the mechanism that Jesus has given us for declaring someone to be a member of his body, and this happens among real people in a real place called the gathering of a local church.