Paul says that preaching the gospel of Christ (what He has done and Who He is) is the Great Speech Act of God (2 Cor. 4:5-6). In the same way that God spoke Light from darkness, so too when the gospel is preached, the same efficacious power is at work.
When the gospel of Jesus is preached, God effects what is proclaimed. The story and accomplishments of Jesus are like the pronouncement of marriage: they effect what they proclaim. The details and glories of the life of Christ are the chisel of the Spirit, engraving the life of Christ in us.
This is a great and glorious promise, but this also means that preaching must be done with great faith. Promises are grasped by faith and not mindless apathy. A sermon without faith is only the ghost of a good sermon (Js. 2:26). But faith makes words become flesh. Faith drives preachers to proclaim what Jesus has done and Who He is in a way that sets Him before the hearers and then sets Him loose on their souls.