But first must he suffer many things, and be rejected of this generation.
-Luke 17:25
Verily I say unto you, This generation shall not pass away, till all be fulfilled.
-Luke 21:32
Just a short study on how to understand "this generation." Of what generation does the Lord speak in these verses and passages?
"geneá" [translated generation] is used 13x in Matthew. The general understanding is a reference to a specific people group. You can look up the references if you wish. A "40-year group" doesn't quite fit.
There are two settings answering two sets of questions in the return of the Lord passages. As with certain things spoken in the Acts age, some things were conditional (e.g. Peter's offer to Israel in Acts 3). We also see in these return verses the Lord's appearance in the clouds:
"They shall see the Son of man coming in the clouds of heaven with power and great glory."-Luke 21:27
If the first part is treated as literal, so must the last.
"And he shall send his angels with a great sound of a trumpet, and they shall gather together his elect from the four winds, from one end of heaven to the other."
-Matthew 24:31
This is suggestive that the prior is conditional. That is, there are other things in this passage that have not come to pass. That is, if the condition is "all be fulfilled," clearly we are not there, this the generation in sight is not in sight. Had Israel repented, perhaps it would have been that generation listening to the Lord, but clearly not all has been fulfilled and the Lord has not returned in the clouds.
"geneá" [translated generation] is used 13x in Matthew. The general understanding is a reference to a specific people group. You can look up the references if you wish. A "40-year group" doesn't quite fit.
Just one example, Matt 11. The group addressed is the specific group in his presence, not a 40-year window of people,
"But whereunto shall I liken this generation? It is like unto children sitting in the markets, and calling unto their fellows And saying, We have piped unto you, and ye have not danced; we have mourned unto you, and ye have not lamented."
He then goes on to condemn them for the rejection of John the Baptist and accusing the Lord of having a devil. The "they" in those verses refers to those people, not a 40-year window of people. "This generation" is to be understood in its context.
There is word left untranslated in the KJV, Luke 21:32. It is the Greek word "ἄν." Some KJV bibles put asterisks there to show its absence since they weren't sure what to do with it. Strong's defines its use: "usually untranslatable, but generally denoting supposition, wish, possibility or uncertainty" [emphasis mine].
More from Strong's: ἄν (1), a particle indicating that something can or could occur on certain conditions, or by the combination of certain fortuitous causes.
The HELPS Word Study notes, "not easily translatable, it always conveys important meaning. (The KJV sometimes translates an as "perchance," "haply"). It further adds, "ἄν is used about 300 times in the NT, introducing statements that have conditional or hypothetical meaning."
So we have this insertion by the Holy Spirit of an uncertainty. Certainly not an uncertainty as to the fulfillment of prophecy, but an uncertainty (from our perspective) as to when, and therefore, as to whom that "generation" who witnesses those things will be.
Let's look at the greater context of Luke 21:32
Then He spoke to them a parable: “Look at the fig tree, and all the trees. When they are already budding, you see and know for yourselves that summer is now near. So you also, when you see these things happening, know that the kingdom of God is near. Assuredly, I say to you, this generation will by no means pass away ἄν till all things take place. Heaven and earth will pass away, but My words will by no means pass away.
We should not miss another clue: the Kingdom of heaven will be near. The Kingdom of Heaven did not come to fruition to the generation the Lord was addressing in Luke. We can thus conclude there is a yet a a generation to come that will witness the Lord's prophetic words coming to pass and it will be they who will not pas away until all things come to pass.