We could just say that the Bible assumes that every Christian will be joined together with a local church, and so the reason you should join a church is because that’s what the Bible teaches. That’s true:
We find that idea in 1 John when it talks about true believers loving other believers (1 John 2:7-11; 3:11-15) and how Hebrews 11:24-25 commands that we not forsake gathering together with other believers. Not to mention that nearly all the NT epistles are written to local churches.
But the Bible gives us more. It not only assumes we will join together with other believers in local churches, but it also gives us reasons why this is best, and we could even say essential, for every Christian.
Ephesians 4:11-16 is one place that the Bible gives us reasons for why it is best, why it is God’s plan, for every Christian to be united to a local church:
1) We are equipped in the local church (4:12).
The reason God gives pastors and teachers to local congregations is to equip you for the Christian life, for the “work of the ministry.” One author describes it like this, ‘pastor’s give you job training so that you can know how to do your job.’ Without being connected to a local church, we will lack the personalized training that God intends for us in order to serve others.
2) We grow toward unity in the faith and knowledge of the Son of God in the local church (4:13).
If you want to know Jesus better, then you need the Church. You cannot go without the church and expect to have a close relationship with Jesus. The church is God’s plan to bring about this work in our lives.
In a world where there is so much division and hostility, if you want to be truly unified with others, then you need the local church. Earlier in the book of Ephesians, Paul has talked about the unity in the church (cf. Eph 4:3-6). And yet we also see throughout this book that this unity doesn’t mean sameness. We have the same beliefs, the same doctrine, but there is diversity ethnicities (Eph 2:11-22), and there is diversity in the gifts which Christ gives (Eph 4:7). The church is meant by God to demonstrate what the kingdom of God will be like, a kingdom where all people will live together in unity and peace. If we want to be a part of unity and peace with others, then we need the church.
3) We are anchored in the truth by the church so that we will not be tossed to and fro by every wind of doctrine, by human cunning, by craftiness in deceitful schemes (4:14)
Do you want to be able to know how to discern whether or not something that sounds really good is actually true and biblical? If you want to not be naive and led astray, then you need the church. Everything the church does is centered around the Word (or should be - we read the word, pray the word, sing the word, and preach the word), and so when we are members of a local church we will grow deep roots in God’s word and be able to know what lines up with it and what doesn’t (cf. Col 2:8)
4) We will hear truth and love spoken to us in the local church (4:15)
We need people who will speak truth and love to us. We need people who will love us enough to tell us the truth. We need people who will not only speak the truth, but do it in a caring way because they want what is best for the other person. This is at the heart of what it means to be a church family.
This is the way the church will build itself up in love. This passage says that being a part of a church is how we grow up in Christ, it’s how we mature in our faith and look more like Jesus. How does it happen? This is how it happens. It happens as we speak to each other in truth and love. If we want to look like Jesus “in every way” (4:15), then we must be willing to love and speak truth about every area of life. We must be willing to hear words of truth and love from others about our lives. If we are unwilling to do this, then we will not grow to look more like Jesus. We will hamstring our spiritual life, we will cap off our growth, we will stay children, and never reach the maturity that God wants for us as individuals and as a church.
5) We grow into mature Christians who look like Christ “in every way” in the local church (4:13, 15-16)
Jesus is working in His church to make her holy and without blemish (Eph 5:25-27). How is Jesus working toward this goal? It is through His word, that this will be accomplished (Eph 5:26), and specifically, it is through the word of God sounding forth within the church - between pastors and members, and between members and members, in all the church does. God has made the church vital in His plan of preparing His people for Himself. The church builds us up to look more like Jesus.
So, why should you join a church? Ephesians 4 teaches the church is God’s plan for us; a family where we will be equipped for the Christian life, where we will love and be loved, and speak and hear the truth in love, so that in every area of our life we will build up each other to know Jesus more, to avoid error, and live lives that look like the fullness of Jesus. If this is what you want, then you should join a church.