If the Book of Acts teaches anything it teaches that the Holy Spirit wills to reach the end of the earth through us. There is no salvation without the witness of a man or woman to Jesus Christ. You and I are an indispensable link in the chain of redemption. But the third lesson is just as important. The Spirit gives power for world-wide witness .
People are indispensable in world missions, but people alone are useless in world missions.
We must have power. Without the power of the Spirit we languish in fruitlessness. The muzzle gets hot when it is fired. When you open your mouth he will fill it. We wait until we know the Spirit dwells in us.