The third purpose of the church as I see it (and I believe the Bible show us) is Service (or ministry, if you prefer). Just as Jesus said of Himself, "the Son of Man did not come to be served, but to serve, and give His life a ransom for many," we are called as Christians to service. The first thing we have to understand is that EVERY Christian is a minister called into service. We have different gifts and talents, but we are expected to use the gifts and talents God has blessed us with to serve Him. It is important to realize that EVERY ministry is important. The best sermon in the world won't help if a couple visiting that Sunday has a bad experience with the Nursery worker. Everyone has a role and they are ALL equally important.
It is also important to realize that there are no "Lone Rangers" in God's kingdom. We are to work together to accomplish what God has planned for us. As we work together, we strengthen each other and protect each other. As we work together, it is so we can reach out and serve others--especially those who don't know God. Why? Because when we willingly serve those who do not know Christ we are building a bridge to lead them to Christ.
If a congregation is not focusing on serving others, that church will simply become friendly to themselves, take care of themselves, and focus all of their resources on themselves. For the body of Christ to be what the Lord has called her to be, we MUST use our gifts and our talents in His service. Too many times we sit back and want to be served and then we complain about the service! Too many times we feel sorry for ourselves. The best way to get out of that is to simply serve others. Remember that we are called to be like Jesus--Jesus who came to serve and to seek those that are lost.