What if human history were such that the Christian church was able to convert every individual on the face of the earth? Vos’s answer:
[I]t is ever necessary to remind ourselves that abrupt eschatology is inherent in the Christian scheme. It was prepared under the auspices of this, born under them, and must in the end stand or fall with the acceptance or denial of them. This is generic eschatology. A simple consideration of the factors in the case suffices to show how indispensable it is. Even if by persistent application of the gradual processes in the most intensive missionary propaganda, it were possible to convert every individual in the world, this would not provide for the conversion of the generations passed away in the course of history, and which none of our means of grace can reach. And, even discounting this, the conversion of all individuals would not make of them perfectly sinless individuals, except one were to take refuge in the doctrine of perfectionism. The sum total of men, therefore, living at any time would, in order to form a perfect world, stand in need of a marvelous soteric and ethical transformation, such as would rightly deserve the name of eschatology. But even this would not exhaust the factors necessary for the establishment of a perfect order of affairs, because the present physical state of the world with its numerous abnormalities, including human physical weaknesses and defects, would render the continuance of such a state of perfection impossible. Thus there would be created as a further element in generic eschatology the need of a transformation of the physical universe, including the resurrection of the body. [Geerhardus Vos, Biblical Theology (Carlisle, PA: Banner of Truth, 2000), 379–80.]
Eschatological redemption is not by works either.
If we fear that this truth puts a damper on missions, we have nothing less than the words of Christ himself to counter those fears and to lead us toward action:
And Jesus came and said to them, “All authority in heaven and on earth has been given to me. Go therefore and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit, teaching them to observe all that I have commanded you. And behold, I am with you always, to the end of the age.” (Matthew 28:18–20)
Not only do we have the words of Christ to spur our mission, but we have Christ himself. Richard Gaffin:
It’s fair also to say that the church has been most captured by the challenge of the Great Commission, especially the largeness of the challenge, the sheer magnitude of the task in view there. The “all”s are quite pronounced: “Disciple all nations,” Jesus says; “teach everything I have commanded.” So, you see, we have a challenge here that is both extensive and intensive, as comprehensive as it is universal…
Promise yourself this before God, that you will never again quote Matthew 28:19 and 20 without verse 18. That you will always emphasize the “therefore.” That you will never overlook that the bookends of the Great Commission are, in fact, its indispensable basis—our Lord Jesus Christ himself. Promise yourself that you will not become so preoccupied with the challenge of the Great Commission that you miss its comfort—the contentment and confidence in Christ it also provides.