I know heaven and earth didn't pass away in 70 AD. You are reading it wrong. Christ was stating that although heaven and earth will one day pass away, His words won't. Then He went on to discuss the conditions leading up to when heaven and earth will eventually pass away and that nobody knows when, but when it does, it will be like the days of Noah.
You understand that the "day of the Lord" happened several times in the OT against different nations.
And the language is similar in the passages.
But we KNOW FOR SURE, that the things described in those passages in the OT., did not literally come to pass,
That is, the stars did not literally fall from the sky, they are still up there.
And did the sun stop shining?, no. did the moon turn to literal blood? no.
So in some ways they must be symbolic.
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The sun is the light of the material world.
God's word and love, is the spiritual "light of the world".
So when we see that it is impossible, for the things to be material, we may turn to the symbolism to understand the meaning. 1 Cor 2:14
When it says that the sun or light became dark, it's saying that the access to the understanding of God is dimmed.
It would be access to God's word, basically the Law.
The people of Israel who were taken away as slaves, lost access to God's word. It could not be read, taught, learned.
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The moon is the Prophets, a reflection of the light.
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When heaven and earth pass away, in symbolism, it's talking again about the covenant relationship with God.
Rev 6:12, The destruction of Jerusalem
Earthquake...A change in the earth's position in it's relationship to the heavens/God.
Sun black....Access to God's word gone, greatly dimmed.
Moon blood....The Prophets, and those who follow their words are killed.
V 13, Stars fall.......Israelites are killed, removed from the high places, in the relationship with God.
Fig tree shaken.....Israel is shaken, The figs are the fruit of Israel.
The tree of Israel was producing fruit, but it was not ripe yet.
The kingdom had begun, but before it was bearing the fruit to it's potential, it was cut off because of unbelief (natural branches)
V 14, The heavens depart as a scroll.....God's Protection was over Israel as long as they were loyal to Him. When Israel rejected Jesus and the kingdom, that protection was removed. It is seen as a scroll, implying, the Law. (When the Christians fled Jerusalem, the Holy Spirit left and the protection over Jerusalem ended, and the Roman armies came "flooding in")
The mountain is the people of Israel in Jerusalem.
The islands are pockets of Israel among the gentile nations. (the waters/seas are gentile nations)
V 15, The kings of the earth are Israelites, the earth is the world of Israel, the earth of Israel, this is written from the perspective of Israel.
V 16, This PROVES that this is the destruction of Jerusalem in 70 ad.
These words were spoken directly to the women of Jerusalem by Jesus Lk 23:30.
This prophecy could ONLY be fulfilled during the lifetime of those women and their children. Those people are dead now, so this cannot be future.
It was fulfilled by 70 ad.
When Jerusalem was destroyed in 70 ad, it PROVED that Jesus was the Messiah, the Lamb.
V 17, The day of His wrath is against Jerusalem, in this passage.
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The symbolism should be maintained through the scriptures, OT and NT. (if possible)