From the famous sermon by John Piper in 1988, Prayer: the Work of Missions:
Not only has God made the accomplishment of his global purposes of salvation hang on the preaching of the Word; he has also made the success of the preaching of the Word hang on prayer. God's goal to be glorified in a world full of white-hot worshippers from every people and tongue and tribe and nation will not succeed without the powerful proclamation of the gospel by people like you and me. And that gospel will not be proclaimed in power to all the nations without the persevering, earnest, global, faith-filled prayers of God's people. This is the awesome place of prayer in the purposes of God for the world. They won't happen without prayer.
Listen to the whole thing.