It’s a popular notion today that one can be a Christian without being a part of the Church. It’s also a thoroughly unbiblical notion.
In The Prodigal God, Timothy Keller has some good thoughts in this regard:
“Christians commonly say that they want to “get to know Jesus better.” You will never be able to do that by yourself. You must be deeply involved in the church, in Christian community, with strong relationships of love and accountability. Only if you are a part of a community of believers seeking to resemble, serve, and love Jesus will you ever get to know him and grow into his likeness.”
I have known plenty of people who were in one sense or another actively involved with the Church who did not particularly resemble Jesus. That’s a common enough claim, and shows that just “going to” church or being involved in church activities doesn’t necessarily conform someone to the image of Christ.
Of course, at the same time, I haven’t known anyone who resembled Jesus who wasn’t also deeply devoted to the Church. Jesus loved the Church enough that he paid for it with his blood (Acts 20.28); to think that one can somehow be a Christian without belonging to that Body is absurd.