That is the question that Greg Gilbert and Kevin DeYoung work toward answering in the book of that title.
I love this book so far. It is clear, informative, and thorough so far. After looking at the Great Commission passages of the New Testament in the Gospels and Acts, they come to this summary statement:
The mission of the church is to go into the world and make disciples by declaring the gospel of Jesus Christ in the power of the Spirit and gathering these disciples into churches, that they might worship the Lord and obey his commands now and in eternity to the glory of God the Father. We believe that this is the mission Jesus gave the disciples prior to his ascension, the mission we see in the New Testament, and the mission of the church today. (emphasis in the original).