No one will be able to figure out the day or the hour, but there is plenty of expectation that believers should be able to read the signs (Mt. 24.32-33; 25.13) and be able to tell (1) it's close, or (2) we're in it. The signs are given to us so that we can recognize when it is upon us.
Matthew 24.13: "So when you see..."
Matt. 24.23-25: "At that time if anyone says to you..., don't believe it. ... See, I have told you ahead of time."
John 14.29: "I have told you now before it happens, so that when it does happen you will believe."
John 16.1: "All this I have told you so that you will not fall away."
The Bible gives us all kinds of clues so that we can see it coming and recognize it when it's here. But the purpose of that understanding is so that we stay faithful and at the same time help other people find God. The point is never to be able to pinpoint His coming, which Jesus says we'll never succeed at doing anyway.
> Is it too bold for a Christian to say that Jesus will probably return by 2100?
It's not too bold, but at this point it's too difficult to tell. I, for one, have spent a lot of time studying the prophecies to understand them. There are many things in Revelation that are just too obscure to understand until they happen, but once they're happening, we should be able to recognize them. Predicting? Not a chance.
On the other hand, there are plenty of things the Bible prophesies that we can see happening before our eyes:
- Natural disasters and increase of cataclysmic phenomena (Mt. 24.7): volcanoes, floods, tornados, earthquakes, hurricanes.
- Political unrest that often focuses on the Middle East
- The increase of communication & knowledge (Dan. 12.4; Mt. 24.30): smart phones, Internet, global communication networks.
- The apostasy of the Church: lots of supposed Christians who just aren't following the Bible or teaching what the Bible teaches (Jn. 5.43; Mt. 24.24). The Church is now filled with people and leaders who teach things the Bible doesn't say, and the Internet is teeming with false information about the Bible, so much so that I often get told, "Well, scholarly consensus disagrees with the Bible." Yeah, of COURSE it does. There are truckloads of false information, misinformation, and downright lies coming from scholars, on the Internet, and deceiving many people.
- The alignment of nations in allied treaties.
But I don't yet see the temple rebuilt in Jerusalem, the anti-Christ, the two witnesses, or many other elements. So it's not upon us yet. It could all coalesce very quickly. For instance, if North Korea were to launch a nuclear bomb, the world could come to devastating war almost overnight. The environment could be ruined, and famine and disease be rampant within months. Economies and supply chains would be in disarray. A world leader could rise, and people ready to follow. One never knows. I don't see any of this happening now, but the world is like a matchbox ready to ignite, just as it was in a smaller sense in 1914. Could it happen this century? Sure. Will it? Who knows. We humans, though, have quite the propensity for self-destruction. Civilization could collapse almost overnight, and, because of our world, it could easily be global and disastrous.