1. The new covenant is in the Old Testament - Jer 31:31-33 it is merely "quoted" in the NT from that one OT text.
2. There is only ONE Gospel Gal 1:6-9 and that Gospel was preached "in the OT" to Abraham Gal 3:8
3. That New Covenant (one Gospel) is what enabled Moses and Elijah to stand "in glory" with Christ in Matthew 17 -- before the cross.
Those texts are actually in the Bible just as noted above.
1. Wrong...
Jer 32:
38 “They shall be My people, and I will be their God;
39 and I will give them one heart and one way, that they may fear Me always, for their own good and for
the good of their children after them.
40 “I will make an everlasting covenant with them that I will not turn away from them, to do them good;
and I will put the fear of Me in their hearts so that they will not turn away from Me.
Quoting other parts of Jeremiah -- even Jeremiah 32 does not make Jeremiah 31 "wrong" or "not the New Covenant" since Hebrew 8:6-11 includes a "verbatim quote" of Jeremiah 31:31-34 insisting that it was true... and still is true.
Jer 31:
31 “Behold, the days are coming, says the Lord, when
I will make a new covenant with the house of Israel and with the house of Judah— 32 not according to the covenant that I made with their fathers in the day
that I took them by the hand to lead them out of the land of Egypt, My covenant which they broke, though I was a husband to them, says the Lord. 33 But
this is the covenant that I will make with the house of Israel after those days, says the Lord:
I will put My law in their minds, and write it on their hearts; and I will be their God, and they shall be My people. 34 No more shall every man teach his neighbor, and every man his brother, saying, ‘Know the Lord,’ for they all shall know Me, from the least of them to the greatest of them, says the Lord. For I will forgive their iniquity, and their sin I will remember no more.”
Now lets find it again in Hebrews 8
“Behold, the days are coming, says the Lord, when
I will make a new covenant with the house of Israel and with the house of Judah— 9 not according to the covenant that I made with their fathers in the day when I took them by the hand to lead them out of the land of Egypt; because they did not continue in My covenant, and I disregarded them, says the Lord. 10
For this is the covenant that I will make with the house of Israel after those days
10 "For this
is the covenant that I will make with the house of Israel after those days, says the Lord:
I will put My laws in their mind and write them on their hearts; and I will be their God, and they shall be My people. 11 None of them shall teach his neighbor, and none his brother, saying, ‘Know the Lord,’ for all shall know Me, from the least of them to the greatest of them. 12 For I will be merciful to their unrighteousness, and their sins and their lawless deeds I will remember no more.”
It is "unchanged" in the NT from its OT form.
It is "the one Gospel" the one and only way of salvation, new birth, adoption, forgiveness of sin, new creation.
2. The Cross justified OT believers in retrospect as it is a timeless event anchored at one point of history.
Which is why that one Gospel of Jeremiah 31 is "unchanged" in OT and NT - it provides for the new creation, forgiveness of sins, adoption into the family of God ..
The result is that before the cross we have Moses and Elijah "in glory" standing with Christ. Fully forgiven, redeemed and in glory.
Matt 17
2 and He was transfigured before them. His face shone like the sun, and His clothes became as white as the light. 3 And behold, Moses and Elijah appeared to them, talking with Him.
Luke 9
29 And while He was praying, the appearance of His face became different, and His clothing became white and gleaming. 30 And behold, two men were talking with Him; and they were Moses and Elijah, 31 who, appearing in glory, were speaking of His departure which He was about to accomplish at Jerusalem.
Luke 9
29 As He prayed, the appearance of His face was altered, and His robe became white and glistening. 30 And behold, two men talked with Him, who were Moses and Elijah, 31 who appeared in glory and spoke of His decease which He was about to accomplish at Jerusalem. 32 But Peter and those with him were heavy with sleep; and when they were fully awake, they saw His glory and the two men who stood with Him
That does not mean OT believers were born again and the indwelling presence prophesied by Jeremiah was not given until after the resurrection.
That statement appears in the form of speculation/preference not a "quote" of something. And you have free will and are welcome to your opinion on that point. We can agree to differ.
3. If the New Covenant was occurring in the OT why would Jeremiah prophesy its coming as a future event?
the same "will make" statement in Jeremiah 31 is also found "unchanged" in Hebrews 8. If that is taken to mean "nobody is under the New Covenant today" then you would be excluding everyone in the OT and the NT - both lived under the same exact unchanged statement.
You would also need "Another gospel" that is not the New Covenant one for OT and one for NT.. that would be "another gospel" and as Paul states that is not a good option since that "ONE gospel" of Gal 1:6-9 was "preached to Abraham".
Hebrews 4 "
for the GOSPEL has been preached to US just as it was to THEM also"
1 Cor 10 "
They all drank from the same Spiritual Rock and that Rock was CHRIST"
Psalms 51 "
taken not Thy Holy Spirit from me"