Based on passages like Colossians 1:23, which says that the whole world heard the gospel in the first century, and Matthew 24, which says Jesus will only come back after everyone has heard the word, isn't it true that we are "safe" from the day of judgment until the whole world has been evangelized? Not to be funny, but if Jesus won't come back till everyone has heard, can't Christians be sure the Lord won't return yet?
Strange question, but a logical one in light of the common interpretation of Colossians 1 and Matthew 24. Colossians 1:23 says every creature under heaven had heard the word. This was written around 61 AD. Are we to take this to mean that the job was finished in the 1st century? That the Chinese had heard the gospel this early? That someone had made it to North America and preached to the native Americans?
In the spirit of Acts 2:5, Colossians 1 is an instance of hyperbole, or deliberate overstatement for effect. (Such a literary device does not need to be taken literally.) For just two of many O.T. examples, see Psalm 98:3 and Jeremiah 25:26. Besides, if Colossians 1:23 were to be understood literally, why would Paul give advice about interactions with outsiders in Colossians 4:2-6?
If anyone wants to be really technical, the term "world" is oikoumene, the same word in Luke 2:1. It refers to the Roman world, not the outlying territories.
Now, I agree that it would be nice if the Lord would delay his coming till all had heard -- after all, doesn't this seem the fairest way to go about things? -- but is this really what the Bible says? Matthew 24 speaks of the end of Israel. Yes, there are many parallels to other judgment oracles. But then again, most judgment oracles in the Bible are delivered against specific nations, and are set in specific historical contexts. That means we have to be careful about generalizing -- making the jump to the end of the world.
The book of Romans also sheds light on this question. In Romans 15, Paul says he looks forward to preaching where Christ has not been made known (esp. Spain); clearly the world has not yet been saturated with the gospel. And yet Romans 10:18 says that the word has (in a poetic sort of sense, based on Psalm 19) gone out to all the world. Proper interpretation entails a willingness to admit when passages are figurative.
One deleterious consequence of this doctrine wrongly understood is the unhealthy pressure it can place on persons and churches to make unwise decisions. The N.T. nowhere commands us to "make disciples of all nations in this your present generation." Not that that wouldn't be a fine thing to shoot for. But when this leads people to unwisely allocate funding and personnel, in the (misguided) belief that the Lord will bless them if they "make it happen" -- plant churches in various parts of the world according to more or less arbitrary criteria of evangelization -- then God's word is not being honored.
Let's get the word out, but let's not add to that word. Indeed, there is a huge difference between a command of God and a good idea. A major difference! While not compromising our zeal, we must fiercely guard the word of God, allowing no on to "go beyond what is written" (1 Corinthians 4:6).