We can understand from the statement of Jesus here that He not only knew the nature and circumstances of His death, but the resurrection and ascension too! These are words of encouragement and challenge. Jesus is saying that if you remain loyal to Him through the darkest times of your life, He will remain true to you. However, if you do not make it clear to others where you stand in faith and matters eternal then you cannot expect a place of honour in God's Kingdom. There is talk elsewhere in the New Testament of some getting in to heaven by 'the skin of their teeth'- 1 Corinthians 3 v 10-15- and this will be the experience of those who are in God's family, but choose to enjoy wealth without conscience and a working life untrammelled by ethical principles.
The second statement here has caused a lot of thought, understandably as the Early Church believed in the imminent return of Jesus Christ and there was always hope in the plain fulfillment of Christ's prophecy until all those who had lived in His time had died; John would have been one of the last. So, do we take it that Jesus was mistaken? We have, however, another verse where Jesus declared that He had no idea when He would return- Mark 13 v 32, 33-, so we should understand this statement to be not about His Second Coming at all. Reasonably, Christ could not possibly have expected to be able to return sixty or so years after His death because one of the prerequisites was that there would be believers in every nation before that was to happen and the timeframe could not have allowed for that! No, what Jesus is talking about is the experience of many millions in discovering God coming in power upon them. In the thirty years after His death, Christianity had swept through Asia Minor, penetrated Egypt, entered Rome and Greece like an unstoppable tide which was transforming the spiritual and ethical and moral standards of society.