In 1 Corinthians 12, Paul compares people in the church to members of a body, and makes the case that the various parts of the body cannot survive if they are disconnected from the body. Paul also describes how we are placed as members in the church by God Himself. Since we cannot survive if we are disconnected from the body, it is required that a Christian be a member in a local church:
1 Corinthians 12:18 But now God has set the members, each one of them, in the body just as He pleased.
It is a terrible distortion of Christianity to believe that Christ died merely for individuals when God’s Word clearly teaches that Christ’s mission was to die for and to build the church or the assembly:
Ephesians 5:25b …Christ also loved the church and gave Himself for her
This doesn’t mean that individuals are unimportant, or that we lose our individual identity once added to the assembly. Contrariwise, it is only within the body that our individuality is made perfect:
1 Corinthians 12:21 And the eye cannot say to the hand, “I have no need of you”; nor again the head to the feet, “I have no need of you.”
1 Corinthians 12:27 Now you are the body of Christ, and members individually.
So it is that we need to be a part of a local assembly in order to grow in Christ.