It was a literal statement that some of that generation in which Christ was manifested on earth would live to see His second appearance promised to that generation. This is a new concept to many, and requires recognition of the time in which His second appearance was promised. (Heb. 9:28) It requires the proper perspective to recognize things which have been deliberately hidden from us for over 150 years.
"God, who at sundry times and in divers manners spake in time past unto the fathers by the prophets, 2 Hath in these last days spoken unto us by his Son, whom he hath appointed heir of all things, by whom also he made the worlds;
3 Who being the brightness of his glory, and the express image of his person, and upholding all things by the word of his power, when he had by himself purged our sins, sat down on the right hand of the Majesty on high:" (KJV)
Hebrews was written approx. 63-66 A.D. (Source: [Hebrews])1 So, the author was telling those Hebrew Christians in the first century A.D. that Christ had spoken with them "in these last days". The author said that in days past, times past, that God had spoken to the people through the prophets, but in the first century He spoke to the people through His son, and the author identified their days as "these last days." A = B = C.
If Christ spoke with the people directly in the first century A.D., and He spoke with them in "these last days" then the last days were in the first century A.D.
This present tense verbs of the New Testament books were all written almost 2,000 years ago to people who lived in the first century A.D. That makes the New Testament history for us, just as the Old Testament is history.
The last days were always the last days, the latter days of the tribes of Israel. See Gen. 49:1-27, where Jacob/Israel was giving the blessings to his sons at his death bed, and told them what would happen to each in their latter days.
The last tribe, the remnant still existing when Christ appeared on earth was Judah, the law giver, in verse 10.
" The sceptre shall not depart from Judah, nor a lawgiver from between his feet, until Shiloh come; and unto him shall the gathering of the people be." (KJV) Bold emphasis mine.
The scepter was removed from Judah and given to Christ at His death on the cross in 31 - 33 A.D. So, the latter days spoken of by Jacob were those days of Christ's crucifixion.
And, the entire book of Hebrews discussed what the latter days were, and what was about to pass away... the old Mosaic sacrificial covenant. The last days were always about the passing away of the old covenant in favor of the new covenant of the gospel of Christ.
" For all the prophets and the law prophesied until John." (KJV)
When did John come? He was born approx. 6 months before Christ was born, and was making the path straight for Christ, preaching that the kingdom was at hand in the first century A.D.! (Matt. 3:2; Mark 1:15)
So all the prophets and the law was until the first century A.D., when Christ told His disciples that they would not finish going through the cities of Israel before He came in His kingdom (Matt. 10:23); when Christ told Caiaphas that Caiaphas would see Him sitting on the right hand of power and coming in the clouds of heaven (Matt. 26:64).
That was judgment language spoken to the High Priest of the Sanhedrin, who recognized it from all OT judgment language of God. That is why they convicted Him of blasphemy, because He spoke judgment against them, and they did not recognize Him as the Son of God.
Christ told them that judgment was going to begin with them, and that the fire had already been kindled (Luke 12:49). John told them that the axe was already laid at the root of the tree (Matt. 3:10). Christ told them that all the blood from Abel to Zacharias would be required of that generation (Matt. 23:35). Christ told them that generation would not pass till all those things were fulfilled (Matt. 24:34).
There are many time texts in the NT that have been ignored in favor of current interpretations of Revelation and Daniel, etc. But, they are easy to recognize once you really look at them.
Christ came back in their generation, bringing with Him the kingdom, judgment, and the glory of victory over His enemies, over all those who had rejected Him as the promised Messiah. It was completed at the destruction of Jerusalem and that temple which He called the synagogue of Satan in Rev. 2:9, 3:9, in A.D. 70.
The only generation which could have a second appearance of Him is the one that saw His first appearance, those that handled Him, walked with Him, witnessed His crucifixion (1 John 1:1-2). We have never seen Him in the flesh. He came back to that very special generation just as He told them He would. He established His kingdom just as He told them He would.
To explore more scripture evidences of the first century appearances of Christ, see the posts "It's Not The End of The World, Parts I - X" at my blog here