2 billion people have never heard Gospel.
131 million people are born each year.
The Gospel is shared with about 250 million new people each year.
At this rate, and using simple mathematics, every person on earth will have heard the Gospel by 2029.
Matthew 24:14 says, "And this gospel of the kingdom will be preached in the whole world as a testimony to all nations, and then the end will come."
Therefore, Jesus could return during our lifetime (for many of us). Wow!
I believe technology will continue to accelerate the advance of the Gospel. However, at the same time, the small people groups that will be the last ones to hear the Gospel, aka the tail-enders, will slow the advance of the Gospel to all nations. I also feel that the Gospel will not just be preached to every person once and right away Jesus will come. I have a feeling there will be a penetration of the Gospel worldwide where everybody sees Christianity as a beacon of light in their own communities. Everyone will be aware of Christians in their midst. I believe before Jesus' return and judgement, God will have adequate time to prove to people worldwide that He is indeed the Lord and He is good. It will be undeniable, even as believers are persecuted and put to death. This is how I see it playing out, but I'm also not an expert on eschatology.