While the “Church” is definitely the acceptable term for the body of Christ, and membership within that body usually refer to being a Christian in general, at least as I sculpt a personal lexicon, the word “Church” could easily be used to refer to a denomination (such as the Catholic Church or the Presbyterian Church) as much as the Christian Church, or it can be used to refer to a congregation, as activity within a local church. In other words, activity within a local church refers to fellowship. Fellowship is essential for spiritual health, as Ricky Jones from Gospel Coalition explains, you should engage within the smaller group to more healthily live as a Christian.
First, it’s simply not possible. To imply you can be part of the greater community without first being part of the smaller is not logical. You cannot be part of Rotary International without also being part of a local chapter. You cannot be part of the universal human family without first being part of a small immediate family.Second, it’s not biblical. Every letter in the New Testament assumes Christians are members of local churches. The letters themselves are addressed to local churches. They teach us how to get along with other members, how to encourage the weak within the church, how to conduct ourselves at church, and what to do with unrepentant sinners in the church. They command us to submit to our elders, and encourage us to go to our elders to pray. All these things are impossible if you aren’t a member of a local church. (See 1 and 2 Corinthians, James, Ephesians, 1 and 2 Timothy, and 1 Peter for references.)