What does 1 Corinthians 12:21 mean?

Scripture has been describing a reason for discontent among some members of Christ's body, the church. Perhaps this was going on in Corinth. Some Christians didn't think the service-role given them by God was valuable or interesting or visible enough. They were discontent with their Spirit-assigned part in the body of Christ. It's possible they did not understand how essential it was to the church for them to fulfill the function God had gifted them. As Paul has stated before, a functioning body must have these diverse parts. It cannot function without them.

Now Paul identifies a separate problem, but one closely related to the first. Some Christians felt they didn't need the service provided by other Christians who were gifted by the Spirit to provide it. They mistakenly believed that they were important to the body of Christ while other believers were unimportant because their gifts did not serve exciting or visible functions. While some were bitter about their spiritual gifts and did not want to use them, these believers were arrogant about their gifts, and didn't think others needed to use theirs.

Paul writes that this is like an eye saying to the hand, or the head saying to the foot, "I have no need of you." Any eye or head that would say such a thing clearly does not understand how bodies work. The brain might think itself more important than the stomach, but it cannot survive without what the stomach does. As Paul explains in upcoming verses, it's often the body parts we sneer at which perform the most vital roles! Any Christian that would think such a thing of a brother or sister in Christ—that their God-given role is unimportant or irrelevant—does not understand how Christ's body works.