The world says, "Join the church of your choice." The Bible says we are added to the church when we are saved (Acts 2:47); thus there is no "joining the church." However, it is to my advantage that I identify myself as a member of a faithful local congregation.
For one thing, this is the example set by the apostle Paul (Phil. 3:17). After his conversion and return to Jerusalem, "he tried to join the disciples; but they were all afraid of him, and did not believe that he was a disciple. But Barnabas took him and brought him to the apostles... So he was with them at Jerusalem, coming in and going out" (Acts 9:26-28). The New Testament does not tell Christians to join a church, but we are to join the disciples; that is - we are to identify ourselves as members of a local congregation.
Secondly, I need to place myself under the charge of a sound eldership. Elders are to watch out for the souls under their oversight (Heb. 13:17). It is their job to feed and protect the flock. I will benefit from this arrangement.
Finally, it is God's design that members be joined together into a body (a local congregation). Read and consider 1 Corinthians 12:12-27. Every member is to be active in the local church, contributing their talents to the benefit of the members and to the overall growth of the church (Eph. 4:16). Our adversary is described as a roaring lion who walks about seeking to devour his prey (1 Pet. 5:8). We know that predators target those who are loners or stragglers. This is exactly what I am if I am a "member at large." "Placing membership" with a local congregation will not make me invincible to Satan's attacks, but joining the disciples will give me an advantage. There is strength in numbers (Eccl. 4:9-12).
Think about it. If you have been attending with us, why not identify with us?