This is a good question often asked, but seldom answered well.  In order to answer this question, it is important to understand what the Bible says about the nature of the church and membership.

 1. Biblical Indications Of Church Membership In New Testament Times

The word church is found throughout the New Testament to refer to the fellowship of believers in a particular area, who of course met together, worshiped together, prayed together, ate together etc. But did people join these local expressions of fellowship in a formal way, or was it a less formal association?  We should note that:

          a. The meaning of the word “join” in Acts 5:13 makes sense only in the context of formal membership. 

In Acts 5:13 we read of the reaction of the non-Christians in Jerusalem after word got out about what happened to Ananias and Sapphira after they lied to the Holy Spirit.  It says,   “No one else dared join them, even though they were highly regarded by the people.”  The same word is used in Acts 9:26 referring to Paul’s attempt to join the disciples following his conversion.  It is not a word that speaks of a casual, come-or-go  relationship.

            b. The instructions for church discipline make sense only in the context of membership.

Jesus plainly expected church discipline to be exercised by the members, (Matthew 18:17) and the apostle Paul says that serious matters should be dealt with when the entire church is “assembled in the name of our Lord Jesus” (1 Corinthians 5:4).  This only makes sense if formal membership is in view, otherwise who has the right to speak and vote on such matters?

            c. The metaphors used to describe local churches (flock, temple, body, household) make sense only in the context of membership.

The New Testament uses several metaphors to describe churches.  Four of these metaphors in particular – flock, temple, body, and household – are used to refer to individual congregations and are word-pictures that expand on the concept of being “joined together” (cf. Acts 20:28; Ephesians 2:21;
1 Corinthians 12:27, 1 Timothy 3:15).  Each of these metaphors is best understood in a setting of specific church membership, for they are words of relationship and community

2. Biblical Reasons For Church Membership

            a. Joining proves that you’re not ashamed to identify with Christ or His people.

Jesus said (in Mark 8:38) that if anyone is ashamed to identify himself with Him on earth then He will not identify Himself with that person when he or she stands before God in the Judgment. Joining a church is one of the plainest ways of saying you’re not ashamed to identify yourself with Jesus and His people.

            b. We stop being independent Christians and place ourselves under the discipline and protection of other Christians.

In Matthew 18:15-17, Jesus talked about accountability and discipline. When a professing believer starts living like an unbeliever, those in the church who know about it are to confront him about his sin.  The goal is to restore the sinner back to full fellowship with the Lord and his or her fellow believers.

            c. We submit as we should to properly instituted authority, obeying God’s command.

“Everyone must submit himself to the governing authorities, for there is no authority except that which God has established. The authorities that exist have been established by God”. (Romans 13:1, NIV).

“Submit yourselves for the Lord’s sake to every authority instituted among men: whether to the king, as the supreme authority…” (1 Peter 2:13, NIV).

            d. We openly demonstrate the reality and unity of the body of Christ.

“Now you are the body of Christ,” Paul wrote to the church at Corinth, “and each one of you is a part of it” (1 Corinthians 12:27, NIV).

But how can we see the body of Christ? When you join a church, you make it visible. You give a living demonstration of the spiritual reality of the body of Christ.

            e. We both encourage and exhort the church when you join the local body of Christ.

“And let us consider how we may spur one another on toward love and good deeds. Let us not give up meeting together, as some are in the habit of doing, but let us encourage one another—and all the more as you see the Day approaching.”  (Hebrews 10:24-25, NIV).

Conclusion

It is the implicit teaching of the New Testament that God desires for believers to formally join with a local congregation as a visible sign of their being joined with Him.