This question is based on a very specific reading of the New Testament favored by certain critical scholars. The reading goes like this:
- Passages in some of the earliest Christian documents (e.g. 1 Corinthians 7:29, 1 Thessalonians 4:13-18, James 5:8) talk of Jesus returning soon.
- Passages in later writings (e.g. Matthew 24-25, Mark 13) talk of Jesus’ return in conjunction with the destruction of the Temple [yes the gospels were written down after the letters of the NT but the material in them is older]
- When Jesus did not return shortly after the destruction of the Temple in Jerusalem (AD 70) Christians needed to rework their theology
This line of reasoning is based on, in part, what happened in 1843, 1845 to William Miller and his followers. Miller had predicted Jesus’ return in both of those years, when it did not happen in 1844, when the dates passed the Millerites (as they were known) suffered The Great Disappointment. However, they soon began interpreting Miller’s predictions as “a spiritual return” and the start of the Millennium and form themselves into the Adventist Church. Some scholars believe this is what must have happened to the earliest Christians when Jesus failed to return in AD 70. However, this fails to answer one basic question, why would Christians write down Jesus’ prediction of his return in AD 70-80 (the dates most scholars believe Matthew and Mark were written)? If he was returning they would have no need to write this stuff down and if he had failed to return then he is wrong and not worth believing on this point, so again why memorialize it in writing?
Obviously, Christians believed Jesus was going to return soon, Christians always believe Jesus will return soon, this is part of how we listen to him when he said, “keep watch” (Matt 24:36-44). However, Jesus’ predictions recorded in Matthew and Mark did not refer to his return. He clearly says he did not know when that would happen (Matt 24:36). Rather, what he was referring to in that prediction was the fulfillment of Daniel 7 where he (the son of man, or human one) would take on the responsibility of ruling the earth along with the Ancient of Days (God the Father). The Temple would be destroyed because God would be located in the Church and not need the Temple any longer, the people of Israel would be scattered from the Land of Israel because all lands would be the new Land and all peoples would be God’s people. This is what happened in the resurrection of Jesus, it was confirmed in AD 70 and so there was no need for Christians to change their beliefs; rather we see in the Gospels and Revelation they double-down on what they had always said. The always believed Christ reigns and he will return some day and now that Israel’s sacred identity has been shifted they saw it more clearly than ever.
The best commentaries on Matthew and Mark explain this though they can be somewhat technical but skimming them can be helpful my recommendations are R. T. France The Gospel of Matthew & The Gospel of Mark