I appreciate how Michael Horton addressed the issue, and I particularly appreciate that he addressed it in the context of a conference examining the implications of the gospel for all of life. Horton:
Our mission is qualitatively different from God's mission. God sends us on a mission, but it's a different mission than the mission he sent his Son on. It's different from the mission he sent his Spirit on. The Son could redeem the world. We can't. Again, loose talk—loose talk in the Church today about our redeeming activity in the world. WE should never, ever sully that wonderful word by saddling it to us as the subject of the verb. When it comes to redeeming anything, we are not the subject of the action. Jesus Christ is. Jesus is the unique, only, exclusive Redeemer of the world.
"Well, we're extending his redemption." No, we're not. There is no extension. He accomplished it once and for all. "Well, we are extensions of his incarnation." No, we're not. We're members of his body. I wasn't born of a virgin. I didn't suffer under Pontius Pilate. I wasn't crucified. I wasn't raised on the third day.
We preach not ourselves but Christ, and we are his ambassadors! Paul says, "Let's get that right. We are not the message. We're the messengers." So our mission is qualitatively different from God's. But because he finished his mission in his Son—his mission of redeeming—and he sent his Spirit to open the hearts of those to whom we speak, we have a mission that is guaranteed success.