Take for example this quote from James V. Brownson's The Promise of Baptism: An Introduction to Baptism in Scripture and the Reformed Tradition. He's answering the question, "What is a Christian?" Brownson first talks about what it means to be an individual Christian:
So Christians are disciples, followers of Jesus who seek to learn and to grow, and who live their lives trusting that God has called and chosen them before they even made their own choice to become disciples. They are thus deeply aware of God's kindness and grace which precedes and empowers their own commitment to Christ. Disciples live by faith, trusting in this grace as the foundation for their lives. (p. 5-6)
He then moves on to consider the wonderful truth about our union Christ, summing up this way: "Christians are always learning and growing toward becoming in their daily lives the kind of persons that they already are in their union with Christ." (p. 7)
Finally, he draws out the corporate implications of what it means to be an individual Christian united to Christ:
Up to this point, we have been discussing what it means to be a Christian. But in a very real sense, there is no such thing as an individual Christian. When God joins Christians to Jesus, God also joins them to something bigger than themselves; they become incorporated into the church, the "body of Christ." In the New Testament, it is inconceivable for Christians to think of themselves as united to Christ without also thinking about the ways they are united to other Christians. This was true even in Jesus' own ministry. He didn't have one disciple; He had twelve, and many more beyond his "inner circle." Almost all of the learning of Jesus' disciples took place as a group, rather than one-on-one interactions with Jesus. This same pattern continued in the early church, as Christians gathered in groups called ekklesiai (the Greek word for "churches," which can also be translated "meetings" or "assemblies"). From the beginning, it was unimaginable that someone might become a Christian without also becoming part of a church, a local gathering of disciples of Jesus. The union with Christ experienced by Christians also unites them to each other. (p. 7)
If we're clear on what the thing is, we're clearer on subsequent questions. If we know what a Christian is beyond "my personal relationship with Jesus," then we're likely to be clearer on the nature and necessity of church membership. I wonder if those who oppose church membership aren't guilty of not having thought enough about what it basically means to be a Christian.