The church is not optional for the Christian: it is the family and body of Christ, the place where the Spirit works through word and sacrament, where gifts serve the whole, and where believers are built up and sent out. Lone Christianity is not biblical Christianity.
Not a summary of these passages: the passages themselves. Read them, then come back to the study below.
The study
What the Scripture says
1
The church is the body of Christ
You are the body of Christ, and members individually. We who are many are one body in Christ, and individually members of one another. The church is not a building or a meeting; it is the people in whom Christ dwells by His Spirit.
Now you are the body of Christ, and members individually.
For even as we have many members in one body, and all the members don’t have the same function, so we, who are many, are one body in Christ, and individually members one of another.
He put all things in subjection under his feet, and gave him to be head over all things for the assembly, which is his body, the fullness of him who fills all in all.
The whole body grows and builds itself up in love as each part does its work. Do not forsake the assembling of yourselves together, but encourage one another, and all the more as you see the day approaching. Growth is communal.
but speaking truth in love, we may grow up in all things into him, who is the head, Christ; from whom all the body, being fitted and knit together through that which every joint supplies, according to the working in measure of each individual part, makes the body increase to the building up of itself in love.
Let us consider how to provoke one another to love and good works, not forsaking our own assembling together, as the custom of some is, but exhorting one another; and so much the more, as you see the Day approaching.
As each has received a gift, use it to serve one another, as good stewards of God's varied grace. To each is given the manifestation of the Spirit for the common good. Your gift was given to build the body, not to sit unused.
As each has received a gift, employ it in serving one another, as good managers of the grace of God in its various forms.
They devoted themselves to the apostles' teaching and the fellowship, to the breaking of bread and prayer. And the Lord added to the assembly day by day those who were being saved. The new life is lived together.
They continued steadfastly in the apostles’ teaching and fellowship, in the breaking of bread, and prayer.
Day by day, continuing steadfastly with one accord in the temple, and breaking bread at home, they took their food with gladness and singleness of heart, praising God, and having favor with all the people. The Lord added to the assembly day by day those who were being saved.
Let the word of Christ dwell in you richly; in all wisdom teaching and admonishing one another with psalms, hymns, and spiritual songs, singing with grace in your heart to the Lord.
Let us consider how to provoke one another to love and good works, not forsaking our own assembling together, as the custom of some is, but exhorting one another; and so much the more, as you see the Day approaching.
but speaking truth in love, we may grow up in all things into him, who is the head, Christ; from whom all the body, being fitted and knit together through that which every joint supplies, according to the working in measure of each individual part, makes the body increase to the building up of itself in love.