In Mark 13:30 Jesus said that “this generation will not pass away” before Jesus comes back at end times. How should I think about this?
This is a fairly common question. If we look at Mark 13, but perhaps even more importantly, at Matthew 24, Jesus is prophesying concerning two different events. The prophecies cover the destruction of Jerusalem in AD 70, and also the second coming of Christ. For example, Matthew 24:15-21 is clearly and, I would say, unambiguously a prophecy concerning the coming destruction of Jerusalem under Titus in AD 70. In fact, Eusebius tells us that the Christian church did indeed flee to Pella at the time of the siege of Jerusalem, escaping the coming judgment on the Jews who had rejected their Messiah. They did so because they understood Jesus to be talking about the events of AD 68-70. The abomination of desolation is a reference back to Daniel and Daniel 8 and the desecration of the temple in 167 BC, but it is also a prophecy of the abomination and the desecration of the temple by Titus when he offered pagan sacrifice to the god of Rome right on the temple site.
So, when Jesus says that the current generation would not all pass away before the events are fulfilled, he is referring to the destruction of Jerusalem, which happened almost exactly forty years after Jesus spoke those words. Some of the apostles, including Peter, had been assassinated by the time of the destruction of Jerusalem, but many, including John, were alive to see the fulfillment of Mark 13 and Matthew 24. This is the explanation of the difficulty.
The problem that some people have is that they find it hard to accept that Jesus and other prophets often gave double prophecies–predictions that had more than one fulfillment, but this double meaning of prophecy is a common pattern in the Bible. Ezekiel 35, 36 and 37 provide examples of this, and there are many more. Matthew 24 and Mark 13 is what I call a double prophecy–it is about two similar comings of God–to destroy Jerusalem, and to destroy the earth by fire at end times. Ezekiel 36 is a prophecy of the return of the Jews to the promised land after 538 BC, but it is also clearly about the church. This is similar to Matthew 24 and Mark 13.