When Jesus speaks, He can only say what is true, because He is the truth (John 14:6) and is full of grace and truth (John 1:14).  He accurately described the horrible events that were to come only 40 years ahead (Luke 21:5-24) when Jerusalem would be invaded by the Romans, destroying everything including the temple.  He also described the events surrounding His second coming (Luke 21:8-11; Luke 21:25-28).  What he said was like a landscape of distant hills in which the nearer hills merged into the distant mountains.  The destruction of Jerusalem and the Second Coming seem to take place at the same time.  But although they are separate, some of the distressful circumstances may well be common to both as in Luke 21:17.  

When Jesus said, "... this generation will certainly not pass away until all these things have happened.", what did He mean by ‘this generation’.  Those who heard Him would experience the terrible downfall of the holy city.  That was the generation listening to Him, but that catastrophic event would prophetically point to the final judgement of the unbelieving world and the final redemption of those who trust in Jesus, including those listening who would die but rise at the great resurrection when Jesus returns (1 Thessalonians 4:13-18).  

Although the earth with other planets and stars appear to be permanent, they are not!  Peter, who believed the truth Jesus spoke, wrote, "By the same (God’s) Word the present heavens and earth are reserved for fire, being kept for the day of judgment and destruction of the ungodly." (2 Peter 3:7).  On the other hand, words which seem to come and go like vapours are all recorded by God - "I tell you that everyone will have to give account on the day of judgment for every empty word they have spoken." (Matthew 12:36).  But the words of Jesus are always true and will always be true - both His words of judgement to those who reject Him, and words of loving affirmation to those who welcome Him (Isaiah 40:8).

Although there are many voices in our world competing to be heard, seeking to influence our daily lives and claiming to predict our destiny, only Jesus is always true.  The effect of His words will continue long after the earth has been remade.  The truth of Jesus' words is not dependent on what humans think of them, or whether culture, national laws or religion approve of them.  What He says is eternally true; the standard against which every other authority is to be measured.  That is why the proclamation of His gospel is so vital.  It is not just another philosophical contribution to human wisdom: it is the power of God to save people for eternity (Romans 1:16).  Although many may disagree, resist and reject what Jesus says, we should remind them that He alone has the last word for the world and everybody on it, and that His word is always true.