I know.
They would know it by the sequence of events Jesus outlines that will precede the son of perdition/abomination of desolation event Jesus describes in verse 20.
Before that event there will be great earthquakes, famines, pestilences, terrifying events in the heavens, the seas and waves roaring, with great distress upon the earth,
Well, all of those things did indeed take place in that generation as recorded in scripture and history.
Jesus
promised His apostles that
they will have famines, wars and rumors concerning wars. This prophecy had special significance during that period of the great Pax Romana ("Roman Peace"), when the outbreak of these wars transpired: Claudius' Roman war with Britain/East Anglia; at least three Jewish insurrections against Rome prior to the 60s AD (one violently put down by Cuspius Fadus); the Jewish/Alexandrian revolt upon Caligula's death; Claudius declares martial law in Palestine after the Jewish insurrection at the death of Agrippa I; the Germanic tribes in present-day Belgium and Germany made perpetual trouble for the legions throughout the reign; a smoldering Balkan war was in continuous progress. As these conflagrations escalated, Rome started its own civil wars in 68-70 that nearly toppled the empire. As Tacitus writes, "Four princes [Galba, Otho, Vitellius, Domitian] killed by the sword; three civil wars, several foreign wars; and mostly raging at the same time. Favorable events in the East [the subjection of the Jews], unfortunate ones in the West. Illyria disturbed, Gaul uneasy; Britain conquered and soon relinquished; the nations of Sarmatia and Suevia rising against us; the Parthians excited by the deception of a pseudo-Nero." For more on wars of this time and false prophets, see: Josephus: Antiquities, 20:5:1-4; 20:8:5-10; Wars, 2:10:1; 2:13:4-7; 6:5:2.
As for famines,
Acts 11:28 records a worldwide famine. Josephus reports famines in Jerusalem in the 60s AD which killed hundreds of thousands during the Jewish War (AD 66-70). There were accounts of infanticide and cannibalism (as foretold in
Deuteronomy 28:53,57) -- Jewish women cooked and ate their babies (Josephus; Wars 6:3:3-4; Wars 5:1:4).
Concerning earthquakes, Seneca writes: "How often have cities in Asia, how often in Achaia, been laid low by a single shock of earthquake! How many towns in Smyrna, how many in Macedonia, have been swallowed up! How often has Paphos collapsed! Not infrequently are tidings brought to us of the utter destruction of entire cities" (Seneca Ad Lucilium Epistulae Morales, trans. Richard M. Gummere, vol. 2, 437). Josephus says of Jerusalem, "the city was besieged on both sides...there broke out a prodigious storm in the night, with the utmost violence, and very strong winds, with the largest showers of rain, with continued lightnings, terrible thunderings, and amazing concussions and bellowings of the earth, that was in an earthquake. These things were a manifest indication that some destruction was coming upon men, when the system of the world was put into this disorder; and any one would guess that these wonders foreshowed some grand calamities that were coming" (Wars, 4:4:5).
There were also verifiable eyewitness accounts of the Many cosmic signs witnessed in the Roman-Jewish war as Fulfillment of the NT eschatological prophesies citing the arrival Horses, chariots, and angelic armies in the clouds, and the Brightness of the Lord falling on the temple itself.
These angelic armies in chariots battling in the clouds, voices, and glorious brightness of God were indeed witnessed at the temple and around Jerusalem as recorded INDEPENDENTLY by Josephus, Tacitus, and the Midrash in these legitimate history books (Josephus, Wars, 6:5:3; 2:22:1-2; 4:4:5; 6:5:2-3; Tacitus, Histories, v. 13; Midrash, Lam 2:11).
70 AD was a foreshadow fulfillment, and Christians actually did flee Judaea then due to remembering Jesus’ warning about seeing Judea surrounded by armies - but after the fact it’s obvious that the preceding events to the son of perdition/abomination of desolation event Jesus described in Luke 21:20, Matthew 24:15, and Mark 13:14, did not occur in 70 AD - which is why we now know it was a foreshadow fulfillment.
As demonstrated above, the events did indeed take place, so the original audience, as you agree, saw them and believed and heeded Jesus warning, correctly.
As for your "shadow fulfillment" claim, there is no such thing. there is Full-fillment, and not fulfillment.
However, Your appeal to "Type/Shadow" brings up an interesting question.
Is this your position concerning ALL prophecy? Can I assume that you believe there will be a future redemption of man through some future sacrifice? In other words, your thought here opens the door for a future Calvary. Perhaps the establishment of a new Church. Another virgin birth?
I have to ask: Where does the bible teach that? Quite simply, it doesn't. That position is entirely unbiblical, and has no root in apostolic belief or teaching (scripture). It is an allegory invented by later futurists, but was unknown to the apostles.
I think you, like so many, fail to understand the difference between Old Testament typology (shadows and types) with the realities of Christ and the New Covenant (which are not mere shadows and types).
Let's start with the crucifixion. What recent events look to be the GREATER fulfillment of the calvary event in our bibles? 9/11?? Columbine? Sandy Hook? Some other event coming soon? Abortion? Something has to fit...rule of multiple fulfillments.
Next, let's look at the virgin birth. Perhaps cloning is the greater fulfillment in our times of that shadow provided by Mary and Jesus? Something has to fit your rule of multiple fulfillments, right?
Christ's ministry of miraculous cures. No doubt that modern medicine is the GREATER messiah of our times for which Jesus was a mere shadow. Modern medicine has brought cures for ailments and diseases on a massive scale NEVER before done by anyone including Jesus. Surely you must maintain that Christ's healings were just a type for a far greater healing of the sick in our times?
Do I really need to keep going?
I have fairly applied the logic presented in your post to other important subjects of bible prophecy to illustrate just how inconsistent and absurd a "multiple fulfillments" concept is.
Where you seem to fail to understand is the O.T. theology of Messianic TYPOLOGY. Typology is the biblically supported doctrine wherein the apostles claim that the Mosaic Ministry (Temple, sacrifices, priesthood, etc) were foreshadowings of Jesus Christ. We all should study this important concept and recognize that
the pattern ends once the Mosaic System is gone.
Please recognize that the terminus for this TYPOLOGICAL method of understanding the Old Covenant passages arrived 20 centuries ago -- and so to continue turning NEW COVENANT ERA things into mere types and shadows of yet-future-to-us events is not Biblically supported.
Typology is a timebound heremeneutic. Those shadows of the O.T. foretold the story of the Messianic Advent via things familiar to the Jews' way of life. Once Christ is come the shadows are done away (Col 2:16-17). The heavenly things that arrived in Christ's advent are the object for which the types only were shadows (Heb 8:4-5). These heavenly New Covenant things are the final destiny. The Law contained the shadow but not the very image of such great things (Heb 10:1-2). We have the heavenly things now. They have been delivered and they are the END product, not the mere means to some future and better end product. The eternal New Covenant
IS the destination. One could say that the entire book of Hebrews, from chapter 1 to chapter 13, argues this exact point. The biblical answer answering this whole question is written down as the book of Hebrews (all).
The shadows are fulfilled in the present, eternal, New Covenant Age that was ushered in during the last days period of the typological Old Testament Age. The New Covenant delivered REALITIES, and not more types and shadows, as appears to be your assertion..
Like many OT types that foreshadowed NT realities, Daniels AoD, even though the Jews understood as fulfilled in Antiochus Epiphenes, Jesus instructs His audience that such OT fulfillment was only typological, and the Antitype was yet future to them....
Luke Extrapolates in the most detail on what/when the disciples would recognize as the fulfillment of the antitype, namely it would be when they saw "Jerusalem surrounded by Armies", and Jesus places further emphasis on the fact that such would be the antitype
fulfillment when He instructs them that those would be "the days of vengeance
in fulfillment of all things written"
While the apostles understood how OT types and shadows pointed to NT fulfillments, they did not in turn teach that NT fulfillments were types and shadows of some future, greater fulfillments.
Again, Do we await another Virgin Birth, another Crucifixion? another 3rd day rising out of the tomb? Do we await some future, greater sacrifice of Christ?..... then why would we await another "Surrounding of Jerusalem by armies in fulfillment of all things written?"
And why only 2? why not 12? or 20 or 200 more?
Once you open the door to "multiple fulfillment" there can be no terminus.
No, scripture does not allow for it, for again, Christ is not the shadow but the object itself. (Col 2:16-17)