As described in Genesis 12, the Jewish people, as descended from Abraham, make up the nation of Israel. The literal, physical nation of Israel is made up of an ethnic people, descended from one man, known as the Jewish people.
But you just said the Jewish people would come to faith in Jesus as their Messiah (or God's anointed one)
Is that not true of all who gain salvation? Faith in Christ, accepting him as a sacrifice for our sis? And if that “sacred secret” has nothing to do with God's dealings with Israel then what does it have to do with? In deed, what is that “secret”?
There is no "secret" - the Gospel is there in plain sight for all who want to see it - and it is God, not I, Who states that the Jewish people will come to faith in Jesus Christ.
The fact that there are Gentiles who take Jesus up on His offer of salvation does not overlap God's purpose and plans for Israel and the Jewish people, nor does that fact replace Israel in God's economy. The two are mutually exclusive, and separate. One has absolutely nothing to do with the other.
As Jesus said he came not to destroy the law but to fulfill it. And Jesus did give the Jews first opportunity to accept him by restricting his preaching to them.
So it was not his rejection the nation that made the way open for Gentiles but the nations rejection of him. They did not want him to gather them together as a hen gathers its chicks. Jerusalem was the “killer of prophets” whose house was abandoned to her and the kingdom taken from her to give to another “nation”. That certainly was a promise kept.
This is not news. The Jews rejected their Messiah, they have been temporarily put aside although there are many Jewish people who have, in this present Church age, come to salvation through faith in Christ, there are promises that God made to the literal, physical nation of Israel that have nothing to do with the Church, and those promises will be kept at a time in the future known as the millennial reign.
The promises God made to the nation Israel and the Jewish people do not apply to the Church.
So how does one become part of this “literal, physical nation of Israel? By converting to Judaism?
No one can "become part of this literal, physical nation of Israel". The nation of Israel that will receive the promises and blessings that God promised them are the generation of Jews who go through the Tribulation and come to faith in Jesus as their Messiah. There is no mention anywhere in Scripture of Gentiles being a part of this literal nation of Jewish believers who will fulfill the New Covenant.
The only chance Gentiles have at salvation is to accept Christ as Savior during this present time, when Jesus makes Himself available for salvation, and during the Tribulation, when people will be saved but will not have the guarantee of eternal security as those who know Christ now have.
No Gentiles will be part of the nation of Israel that exists during the millennial kingdom and which has been saved and reconciled to their God and who will, as God states, have His law written on their hearts.
And to "convert to Judaism" at this point in time is to reject Jesus Christ as the Messiah and is not advisable.
From the city of Jerusalem in the Middle East or from heaven?
The exact land that will be given to the Jews is detailed in Genesis 15:18-21:
And it came to pass, when the sun went down and it was dark, that behold, there appeared a smoking oven and a burning torch that f passed between those pieces. 18 On the same day the LORD g made a covenant with Abram, saying: h "To your descendants I have given this land, from the river of Egypt to the great river, the River Euphrates - 19 "the Kenites, the Kenezzites, the Kadmonites, 20 "the Hittites, the Perizzites, the Rephaim, 21 "the Amorites, the Canaanites, the Girgashites, and the Jebusites."
As we know, Israel does not at this present time possess all the land that God promised them, and we know from Scripture that when Israel fulfills the New Covenant during the millennial kingdom, they will be given and live in the land, not Heaven, that God promised them.
In addition, we know from Scripture that Jesus will rule from Jerusalem during His millennial kingdom:
16 And it shall come to pass that everyone who is left of all the nations which came against Jerusalem shall go up from year to year to worship the King, the Lord of hosts, and to keep the Feast of Tabernacles.
17 And it shall be that whichever of the families of the earth do not come up to Jerusalem to worship the King, the Lord of hosts, on them there will be no rain.
18 If the family of Egypt will not come up and enter in, they shall have no rain; they shall receive the plague with which the Lord strikes the nations who do not come up to keep the Feast of Tabernacles.
19 This shall be the punishment of Egypt and the punishment of all the nations that do not come up to keep the Feast of Tabernacles.
20 In that day "HOLINESS TO THE LORD" shall be engraved on the bells of the horses. The pots in the Lord's house shall be like the bowls before the altar.
21 Yes, every pot in Jerusalem and Judah shall be holiness to the Lord of hosts. Everyone who sacrifices shall come and take them and cook in them. In that day there shall no longer be a Canaanite in the house of the Lord of hosts. (Zechariah 14:16-21)
Surely you know that those who adhere to faith are the ones who are Abraham's seed, said Paul. (Gal.3:7)
Thus being Greek, Jew, slave, freeman, male, female was of no matter, you are all one person in Christ. (Gal. 3:28)
Moreover if you belong to Christ you are really Abraham's seed, heirs of a promise.(Gal. 3:29)
Paul thus identifies who makes up that nation that was to come from Abraham and being Jewish or not was irrelevant.
“There is no secret”? Paul said there was and he spoke by God's spirit did he not?
Paul spoke by revelation from God and it God's view point he was giving about who was being recognized now as a “seed of Abraham”.
And that sacred secret?
(Eph. 3:5,6) Which in other ages was not made known unto the sons of men, as it is now revealed unto his holy apostles and prophets by the Spirit.
That the Gentiles should be fellow heirs, and of the same body, and partakers of his promise in Christ by the gospel:
After so saying Paul applies the promises made to Abraham and Isaac to the Jerusalem above, not to the then existing fleshly nation of Israel.
So being part of that “literal, physical nation Israel” meant nothing when it comes to who are children of God,
” the children of the flesh are not really the children of God, but the children of the promise are counted as the seed.” (Romans 9:8)
“No Gentiles will be part of the nation of Israel that exists during the millennial kingdom and which has been saved and reconciled to their God and who will, as God states, have His law written on their hearts.”
Paul disagrees. (Romans 9:8)
“The fact that there are Gentiles who take Jesus up on His offer of salvation does not overlap God's purpose and plans for Israel and the Jewish people, nor does that fact replace Israel in God's economy. The two are mutually exclusive, and separate. One has absolutely nothing to do with the other.”
Again Paul disagrees. That which God calls Israel is made up of fleshly Jews and Gentiles and this Israel is heir to the promises. (Eph. 3:5,6)
“As we know, Israel does not at this present time possess all the land that God promised them, and we know from Scripture that when Israel fulfills the New Covenant during the millennial kingdom, they will be given and live in the land, not Heaven, that God promised them.”
Paul wrote that “the Jerusalem above is free and she is our mother” (Gal.4:26) Hence their “citizenship exists in the heavens” (Phil 3:20) “You have approached a Mount Zion and a city of the living God, heavenly Jerusalem and myriads of angels.” (Heb. 12:22)
“Israel at this present time” was the result of U.N. mandate not any covenant relationship with God unless you are trying to say that nation is a continuation of the ancient Israel or the Israel of Jesus’ day.
It is from heaven that Christ will rule as the promise made to David is fulfilled. Christ sits at the right hand of God. (Acts 2:29-35)
Time after time the Scriptures make clear God will keep all his promises to Israel and shows us who this Israel is as defined by God.