Here is something most Christians don't know:
Jesus promised no less than FOUR times in the gospels that he would return to earth and his apostles would live to see it, yet he never showed up.
Paul believed fervently that Jesus would return in his lifetime:
"For the Lord himself shall descend from heaven with a shout, with the voice of the archangel, and with the trump of God: and the dead in Christ shall rise first: Then WE who are alive and remain shall be caught up together with them in the clouds to meet the Lord in the air. And thus we shall always be with the Lord."
1 Thessolonians 4:16-17
Now Paul may have put this idea into the heads of the gospel writers when they started writing the gospels. They had Jesus make these four prophesies that he would return before the apostles died:
“For the Son of Man is going to come in the glory of His Father with His angels, and will then repay every man according to his deeds. Truly I say to you, there are some of those who are standing here who will not taste death until they see the Son of Man coming in His kingdom.“
(Matthew 16: 27, 28)
For me, this is the one that cannot be excused away with rationalizations like "Jesus was referring to the future generation", or "Jesus was referring to God's time which could be thousands of years in the future". We have Jesus referring directly to the people listening to him when he made that failed prophecy "some of you standing here will not taste death until you see me return"
...they will see the Son of Man coming on the clouds of the sky with power and great glory. And He will send forth His angels with a great trumpet and they will gather together His elect from the four winds, from one end of the sky to the other. Truly I say to you, this generation will not pass away until all these things take place.“ (Matthew 24: 25-34)
“Then they will see the Son of Man coming in clouds with great power and glory. And then He will send forth the angels, and will gather together His elect from the four winds, from the farthest end of the earth to the farthest end of heaven. Truly I say to you, this generation will not pass away until all these things take place…“ (Mark 13:26-30)
Here in Chap 10 Jesus is giving his apostles instructions on how they should conduct themselves when he sends them out to do God's work
Truly I tell you, you will not finish going through the towns of Israel before the Son of Man comes. (Matthew 10:23)
Naturally Jesus never showed up. Why? Because we can assume he never said he would return--this was all invention by the gospel writers probably based on Paul's belief Jesus would return and Paul would live to see it. There are five rationalizations Christians have come up with for Jesus' no-show. All are patently ridiculous but you can read them in the link below:
Jesus’ Failed Prophecy About His Return