The local church is a believer’s immediate access to “plugging in” to the body of Christ. Just as each Christian is a small yet vital member of the vast church of Christ, each local church is a unique and important part of the global church. The human body is an excellent illustration to how that works.
The human body as a whole is made up of quite a few different “systems” such as the circulatory system, respiratory system, reproduction system, etc. Each of these systems is made up of various organs, which are comprised of combined tissues, which are maintained by individual cells. Each entity, from the entire body down, is both singular and unique in itself, yet is comprised of equally singular and unique entities.
The church of Christ is (or at least should be) very similar in concept and function. The key to maintaining it is unity and cooperation amongst each unit. This finally brings me to my point. The best way to ensure unsuccessfulness in the body of Christ is to create disunity amongst its members, and vice versa. Contention amongst individuals in a local church is a starting point for the opposing forces of evil, and it can be achieved in a number of ways.
The most basic and obvious source that I know of is a simple difference of opinion or interpretation of truth, where each member firmly posits his position as necessary for everyone else to agree with. Obviously, not everyone does, and tension ensues. Disunity is not isolated to within the local church, of course. Local churches argue with other local churches even within denominations, and the tension between denominations is on a massive scale! The satan has indeed used this weakness to his advantage, and it is one every Christian should be well aware of.