So your really important question is this:
"Why did Jesus cause and allow the early Christians to be mistaken about the timeframe of His return?" Let's figure that out, before we attempt to answer this headline question: "Why did the early Christians think Jesus would return soon?"
I've often wondered about what Jesus was saying in Matthew 24:36 & Mark 13:32 during Jesus' 2nd coming.
Matthew 24:36 (NKJV): “But of that day and hour no one knows, not even the angels of heaven, but My Father only.”
Mark 13:32 (NKJV): “But of that day and hour no one knows, not even the angels in heaven, nor the Son, but only the Father.”
The Bible informs us that God knows everything, thus making Him omniscient.
1 John 3:20 (NKJV): "For if our heart condemns us, God is greater than our heart, and knows all things."
The Bible also tells us that Jesus is God.
John 1:1 (NKJV) "In the beginning was the Word, and the Word was with God, and the Word was God."
John 20:31 (NKJV) "but these are written that you may believe that Jesus is the Christ, the Son of God, and that believing you may have life in His name."
Now that we've found where the Bible states that God knows all things & that Jesus is God, then how does Jesus (who is God)... not know the day, nor the hour?
I came up with these ideas:
- Jesus who is God is also a human on Earth, before He died & rose again. That's how He identifies with us. So when He was born & growing up, He wouldn't have known when He'd return the 2nd time (or 3rd time, if you count His resurrection as the 2nd time). He may have had to wait until after He ascended into Heaven to learn that information from God the Father.
- God the Father may not yet have chosen which day nor hour, that the return will happen. If He hasn't yet decided, then nobody knows. Thus Jesus' quotes in Matthew 24:36 & Mark 13:32 would still be 100% accurate. (Don't quote me on that idea, but at least it satisfied my initial curiosity in the past about those 2 verses. It freed me from worrying about them & allowed me to move on towards learning new things.)
- From reading the Bible, I've discovered some more details... like God is patient & He is still waiting for Israel to do something. (This sounds like the most accurate of these 3 ideas. Let me explain why....)
2 Peter 3:9 (NKJV) "The Lord is not slack concerning His promise, as some count slackness, but is longsuffering toward us, not willing that any should perish but that all should come to repentance."
Part of the problem appears to be that the Jewish Pharisees & Scribes of the law didn't want to honor Jesus as God. They should have known when Jesus was coming the 1st time:
Luke 19:41-42 (NKJV): "Now as He drew near, He saw the city and wept over it, saying, “If you had known, even you, especially in this your day, the things that make for your peace! But now they are hidden from your eyes."
The people did acknowledge & honor Jesus as God in Matthew 21:9 (on the 1st Palm Sunday), but the religious leaders did not. (But the people wanted a king who would free them from the Roman occupation. They weren't looking for a new religious leader... God. They wanted a military king, like David.)
Matthew 21:9 (NKJV): "Then the multitudes who went before and those who followed cried out, saying:
“Hosanna to the Son of David!
‘Blessed is He who comes in the name of the Lord!’
Hosanna in the highest!”"
Then Jesus says,
Matthew 23:39 (NKJV): "for I say to you, you shall see Me no more till you say, ‘Blessed is He who comes in the name of the Lord!’"
Now before this in verse 29, Jesus starts off with: “Woe to you, scribes and Pharisees, hypocrites!...”
Apparently, God is still waiting for Israel (especially, the religious elite in Jerusalem... today known as rabbis) to acknowledge & honor Him as God. The end of the world won't happen, before that point.
The Hebrew leaders who left Egypt during the Exodus, saw God with their own eyes... before having a feast in front of Him:
Exodus 24:9-11 (NKJV): "Then Moses went up, also Aaron, Nadab, and Abihu, and seventy of the elders of Israel, and they saw the God of Israel. And there was under His feet as it were a paved work of sapphire stone, and it was like the very heavens in its clarity. But on the nobles of the children of Israel He did not lay His hand. So they saw God, and they ate and drank."
So initially, God was happy with His people. :-)
The early Christians could have been mistaken about the timeframe of Jesus' 2nd coming, as they didn't have access to personalized copies (nor online versions) of the Bible, where they could study for themselves. They had scrolls, auditory knowledge from preaching & shared letters. Today, we have the ability to do our own Bible study homework, both in print form & online.
Now, Zechariah 14 & Amos 5:16-27 both talk about the Day of the Lord, which hasn't come yet. You can read those if you'd like to do that.
So what about this question? "Why did the early Christians think Jesus would return soon?"
During Jesus' day, there was a temple in Jerusalem. It was the 2nd temple. At the end of the world, there will be another temple, the 3rd temple. Here are some links about that 3rd temple, which contain additional Bible verses in these articles:
In Jesus day, they had a temple. So it would have been easy for them to assume, that Jesus could return at any time.
After reading Amos 5, it seems clearer that God doesn't want the 3rd temple's sacrifice system:
Amos 5:22 (NKJV): "Though you offer Me burnt offerings and your grain offerings, I will not accept them, Nor will I regard your fattened peace offerings."
After the temple was destroyed in AD 70, there hasn't been a temple in Israel. Most of the people were older or dead at that point. So they would have realized the need to write everything down.
Revelation appears to have been written after the temple was destroyed & before AD 96. See:
- When Was the Book of Revelation Written?
- Wikipedia: Revelation
I would assume that some people wanted Jesus' return to happen early - in their lifetimes - for the same reason that people today want the "Rapture" to happen early in our lifetimes. People are terrified of death. The problem isn't death itself, but what lies beyond death. For the believers, it's an eternal happiness in Heaven. For the unbelievers, it's a firey torturous experience in hell. People are afraid of going into hell, so they tend to want Jesus to return early to avoid death & hell.
John talks about Heaven, starting with Revelation 4. Revelation, gives us hope that a brighter future is coming! Here is some encouragement to brighten your day:
Acts 16:31 (NKJV): "So they said, “Believe on the Lord Jesus Christ, and you will be saved, you and your household.”"
Romans 10:9 (NKJV): "that if you confess with your mouth the Lord Jesus and believe in your heart that God has raised Him from the dead, you will be saved."
So don't worry too much about when Jesus will return, but focus more on the quality of your relationship with God. Keep reading your Bible. The more times, the better! You'll find out in Heaven, that you'll gain eternal rewards from continually reading it. If you'd like to see a bit of that before then, check out these links, about which crowns that believers can receive in Heaven:
Hopefully, that answers some of your questions! :-)