Believe, then belong. However, it is certainly possible that one might attend, then believe, and then belong. That is to say, church membership is for those who have a credible profession of faith (and some would add as well that the children of those who have a credible profession of faith should likewise be members of the church.) It is not a civic association, a country club, or any such thing. It is a local body of professing believers in the finished work of Christ.
Attendance is another thing altogether. While we believe that corporate Lord’s Day worship is designed to be the assembling together of the saints, and not an evangelistic event per se, at the same time the gospel is made known, or should be, when the saints gather. The Puritans wisely believed not that “going to church [will] eventually lead to conversion,” but that God the Holy Spirit is far more likely to give new life to a man sitting under gospel preaching than a man sitting on a bar stool Sunday morning. It is always a wise thing to sit under the faithful preaching of the Word of God. Faith, after all, comes by hearing.
If God has indeed given a man new life, his immediate obligation is to be baptized (if he has not already been baptized) and to come under the authority of a local body of believers. He ought not to wait for there to be the desire, but ought to be instructed that such is his calling. When we join a local church our faith is nurtured and fed, both in Word and sacrament. We are protected by the grace of church discipline. And we are given an opportunity to serve the body as the Spirit equips us for ministry.
One of the great successes of the serpent in our own day is that he has persuaded too many of us that joining a local church is unnecessary and superfluous. Many claim to be members of “the invisible church.” Others argue that church membership vows are not in the Bible. That is true enough. The Scriptures do, however, call us to submit to those in authority over us, even those who will give an answer for our souls (Hebrews 13). If you are willing to publicly acknowledge your obedience to that particular command, you have joined a church.
On the other hand, obviously church membership will save no one. One of the dangers of the view that one can belong first and then believe is the temptation to believe that belonging is what matters. Too many of us have said of this loved one or that, “Well, he’s not a Christian, but at least he attends a church.” If such a man does not believe, his membership in the body will only bring greater judgment, especially if he profanes the Lord’s Table by eating there without saving faith.