The church is a body, not a building; an organism, not an organization.
For the organs of your body to fulfill their purpose, they must be connected to your body. The same is true for you as a part of Christ's Body. You were created for a specific role, but you will miss out on this if you're not attached to a living, local church. You discover your role in life through your relationships with others.
The Bible tells us, "Each part gets its meaning from the body as a whole, not the other way around. The body we're talking about is Christ's body of chosen people. Each of us finds our meaning and function as a part of his body. But as a chopped-off finger or cut-off toe we wouldn't amount to much, would we?" (Romans 12:4-5 MSG)
If an organ is somehow severed from its body, it will shrivel and die. It cannot exist on its own, and neither can you. Disconnected and cut off from the lifeblood of a local body, your spiritual life will wither and eventually cease to exist. (Ephesians 4:16)
This is why the first symptom of spiritual decline is usually inconsistent attendance at worship services and other gatherings of believers. Whenever we become careless about fellowship, everything else begins to slide, too.
Membership in the family of God is neither inconsequential nor something to be casually ignored. The church is God's agenda for the world. Jesus said, "I will build my church, and all the powers of hell will not conquer it." (Matthew 16:18 NLT)