Therefore since we are God's offspring, we should not think that the divine being is like gold or silver or stone-- an image made by man's design and skill. In the past God overlooked such ignorance, but now he commands all people everywhere to repent. For he has set a day when he will judge the world with justice by the man he has appointed. He has given proof of this to all men by raising him from the dead.
The important message that Paul is driving home here is God's command, which is "to all people everywhere." That message is a fairly simple one: repent. It's the same message that Jesus preached himself, "repent, for the kindgom of heaven is near." The fundamental Gospel message has to do with repentance, with turning back to God and away from living according to our own desires. It means being reconciled to God through the forgiveness of sins. But how do we know this message is true out of all of the conflicting messages that claim to be from God?
The answer to that (if you haven't already guessed it) is the Resurrection itself. Paul says the Resurrection is the proof that God has given that this message is the truth. No other purported message from God comes with this kind of empirical test of its veracity. Nothing else even comes close.
But how does God intend to communicate this message of repentance to all people everywhere? The answer is, of course, through missions, and through the preaching of missionaries. In Romans 10:13-15 we read:
"Everyone who calls on the name of the Lord will be saved." How, then, can they call on the one they have not believed in? And how can they believe in the one of whom they have not heard? And how can they hear without someone preaching to them? And how can they preach unless they are sent? As it is written, "How beautiful are the feet of those who bring good news!"
We must be careful to guard against any idea of missions that takes away from this one fundamental task: to proclaim God's message of repentance and forgiveness through Christ to "all people everywhere." The task of proclaiming God's message to the world has been entrusted to the church. It is a task that we must fufill.