The mission of the church is eternal. Its origin is in eternity and its destination is eternity. God's plan of redemption for this fallen world was not an afterthought or an expression of a plan B. Rather, before the world was even created, in all eternity God the Father, God the Son and God the Holy Spirit had a covenant among themselves that we call the Covenant of Redemption.
It was God's eternal design in eternity to manifest His plan of redemption and the creation of His church. The Father from eternity agreed to send the second person of the Trinity, the Son, into the world to accomplish this plan of redemption. Together, the Father and the Son agreed to send the Holy Spirit to apply the accomplished work of redemption to God's people.
So the mission of the church begins with the mission of the second person of the Trinity. A mission involves a sending. The Father sends the Son into the world in order to effect God's eternal plan of redemption. That mission is accomplished by the Son. As His mission was accomplished He commanded His people, those who believed in His name, to go into all the world, to proclaim the gospel to all people, that the Kingdom of God may be made known throughout the earth and throughout the ages. The mission of the church began with a mandate given to the church, by the One whose mission was to fulfill all things that the Father sent Him to do. So He said, "As the Father sent Me so send I you." Looking backward we see that the mission of the church began in eternity with the agreement in the Godhead among the Father, the Son and the Holy Spirit and the end of that mission is made manifest in the eternal redemption that God's people experience.
The Kingship of Christ is one that lasts forever. He is the King of Kings, the Lord of Lords and He shall reign forever and ever. In that eternal reign His church, that is made up of His people will join in His triumph and participate in His glory as the kingdom of Christ lasts forever. We see then that there is no end to the mission of the church other than its eternal destiny of salvation and glory. We need to understand that the mission of the church is not a temporary matter. It's not a concern that merely began in Old Testament days and reached its conclusion at end of the old creation. It is a mission from the ages and unto the ages and for the ages. The church must understand that her mission is one that matters eternally. It is a mission that can never be regarded as being of secondary importance.
The seeking of the kingdom of God is the central business of the church. Our Lord commanded that we should seek first the kingdom of God and His righteousness and all other things will be added unto us. So there is no higher priority for the church then the making manifest and bearing witness to the Kingdom of our Savior. This is an eternal destiny with eternal consequences and is of eternal importance. Everything that the church does today and tomorrow matters and counts forever.
This excerpt is taken from R.C. Sproul's contribution in The Mission of God Study Bible. (Holman Bible Publishers, 2012)