> "...I will make a NEW Covenant with the
> house of Israel, and the house of
> Judah...For this is the Covenant that I WILL
> MAKE [future tense] with the house of Israel
> AFTER THOSE DAYS (what are those days ?),
> saith the Lord;

To find out what "those days" are and to understand the verb tense, and the references to both Israel and Judah, you have to realize that Paul was quoting an OT passage.

The future verb tense is from the OT prophecy. When Paul quotes it several hundred years later he quotes the future tense from the prophecy, but in fact the prophecy refers to events that were fulfilled in Jesus and are therefore already past events to Paul.

The actual passage is from Jer. 31. But to fully understand and put it into context its good to go a couple of chapters further back. In Chapter 29 Jeremiah is writing soon after Judah was carried away captive to Babylon by Nebudchanezzar. He's writing to some of the captives.

Jer 29:1 ¶ Now these are the words of the letter that Jeremiah the prophet sent from Jerusalem unto the residue of the elders which were carried away captives, and to the priests, and to the prophets, and to all the people whom Nebuchadnezzar had carried away captive from Jerusalem to Babylon;

The Lord speaks through Jeremiah to tell the captives not to rebel against the captivity, but to be at peace with their captors, and He promises to bring them back to Palestine after 70 years.

Jer 29:4 Thus saith the LORD of hosts, the God of Israel, unto all that are carried away captives, whom I have caused to be carried away from Jerusalem unto Babylon;
5 Build ye houses, and dwell in them; and plant gardens, and eat the fruit of them;
6 Take ye wives, and beget sons and daughters; and take wives for your sons, and give your daughters to husbands, that they may bear sons and daughters; that ye may be increased there, and not diminished.
7 And seek the peace of the city whither I have caused you to be carried away captives, and pray unto the LORD for it: for in the peace thereof shall ye have peace. .... 10 For thus saith the LORD, That after seventy years be accomplished at Babylon I will visit you, and perform my good word toward you, in causing you to return to this place.

God tells the captive Judaites that they are actually blessed to be safe in captivity, because He intends to destroy those Judaites remaining in Judah who were not carried away.

Jer 29:16 Know that thus saith the LORD of (concerning) the king that sitteth upon the throne of David, and of all the people that dwelleth in this city, and of your brethren that are not gone forth with you into captivity; 17 Thus saith the LORD of hosts; Behold, I will send upon them the sword, the famine, and the pestilence, and will make them like vile figs, that cannot be eaten, they are so evil.
18 And I will persecute them with the sword, with the famine, and with the pestilence, and will deliver them to be removed to all the kingdoms of the earth, to be a curse, and an astonishment, and an hissing, and a reproach, among all the nations whither I have driven them:

In chapter 30 Jeremiah prophesies that once they have been punished enough so that they remember the Lord, God will bring the captives back from captivity and restore them into the land.

Jer 30:3 For, lo, the days come, saith the LORD, that I will bring again the captivity of my people Israel and Judah, saith the LORD: and I will cause them to return to the land that I gave to their fathers, and they shall possess it...

Jer 30:10 ¶ Therefore fear thou not, O my servant Jacob, saith the LORD; neither be dismayed, O Israel: for, lo, I will save thee from afar, and thy seed from the land of their captivity; and Jacob shall return, and shall be in rest, and be quiet, and none shall make him afraid. ...

Jer 30:15 Why criest thou for thine affliction? thy sorrow is incurable for the multitude of thine iniquity: because thy sins were increased, I have done these things unto thee. ...

Jer 30:17 For I will restore health unto thee, and I will heal thee of thy wounds, saith the LORD; because they called thee an Outcast, saying, This is Zion, whom no man seeketh after.
18 ¶ Thus saith the LORD; Behold, I will bring again the captivity of Jacob's tents, and have mercy on his dwellingplaces;...

22 And ye shall be my people, and I will be your God.

This same prophecy continues in Chapter 31.

Jer 31:1 ¶ At the same time, saith the LORD, will I be the God of all the families of Israel, and they shall be my people.
2 Thus saith the LORD, The people which were left of the sword found grace in the wilderness; even Israel, when I went to cause him to rest.

The Lord begins to describe how he will bring back the captives from Babylon and from the other countries where they have been scattered.

Jer 31:7 For thus saith the LORD; Sing with gladness for Jacob, and shout among the chief of the nations: publish ye, praise ye, and say, O LORD, save thy people, the remnant of Israel. 8 Behold, I will bring them from the north country, and gather them from the coasts of the earth, and with them the blind and the lame, the woman with child and her that travaileth with child together: a great company shall return thither....

10... He that scattered Israel will gather him, and keep him, as a shepherd doth his flock....

Jer 31:27 ¶ Behold, the days come, saith the LORD, that I will sow the house of Israel and the house of Judah with the seed of man, and with the seed of beast.
28 And it shall come to pass, that like as I have watched over them, to pluck up, and to break down, and to throw down, and to destroy, and to afflict; so will I watch over them, to build, and to plant, saith the LORD....

28 And it shall come to pass, that like as I have watched over them, to pluck up, and to break down, and to throw down, and to destroy, and to afflict; so will I watch over them, to build, and to plant, saith the LORD.
29 In those days they shall say no more, The fathers have eaten a sour grape, and the children's teeth are set on edge.
30 But every one shall die for his own iniquity: every man that eateth the sour grape, his teeth shall be set on edge.

Notice that promise here. God says that people will no longer suffer for the sins of others, but for their own sins. In other words God will deal with every person individually. Not as a part of a group.

Now here's the part Paul was quoting in Hebrews.

Jer 31:31 Behold, the days come, saith the LORD, that 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 bring them out of the land of Egypt; which my covenant they brake, although I was an husband unto them, saith the LORD:
33 But this shall be the covenant that I will make with the house of Israel; After those days, saith the LORD, I will put my law in their inward parts, and write it in their hearts; and will be their God, and they shall be my people.
34 And they shall teach no more every man his neighbour, and every man his brother, saying, Know the LORD: for they shall all know me, from the least of them unto the greatest of them, saith the LORD: for I will forgive their iniquity, and I will remember their sin no more.

So to resume:

After what days?
After the 70 years of exile, and after God brings the captives back from the various countries where they were to be scattered.

And what would God do then?
He would make a new covenant with all his people.

That includes:

not only JUDAH
(which had, in Jeremiah's day just been invaded and most of the people carried away as captives).

But also ISRAEL
The kingdom of Israel made up of 10 of the 13 tribes had been carried away captive by the Assyrians over a hundred years earlier and they had been dispersed into all the earth and had totally ceased to exist an any kind of earthly people. They never did reappear as an earthly people.

So the only possible interpretation of "Israel" in Jeremiah's prophecy is the "totality of God's people". In that sense, in the old testament, Israel included all 13 tribes.

But let's not forget the verse about God sowing Israel with the "seed of men"? (gentiles)

Because of this and other prophecies that mention the return of Israel (the ten lost tribes) the Jews of Jesus' time though that the messiah would somehow bring the missing tribes back to Palestine.

What they didn't understand was that the promise was never to flesh Israel. Even from the very start, the promises of God were always to those of faith, "spiritual Israel".

Jeremiah probably didn't understand that he was prophesying about Gentiles and Jews together becoming Spiritual Israel through faith in Jesus.

But Paul (several hundred years later) certainly understood it all and the reason he quotes that passage is to show that the creation of the church and it's place as the true spiritual Israel of God were prophesied by the Old Testament prophets.

The true Israel is only spiritual Israel.

Ro 2:28 For he is not a Jew, which is one outwardly; neither is that circumcision, which is outward in the flesh: 29 But he is a Jew, which is one inwardly; and circumcision is that of the heart, in the spirit, and not in the letter; whose praise is not of men, but of God.

1Co 15:50 Now this I say, brethren, that flesh and blood cannot inherit the kingdom of God; neither doth corruption inherit incorruption.

Ga 3:7 Know ye therefore that they which are of faith, the same are the children of Abraham.

Ga 3:29 And if ye be Christ's, then are ye Abraham's seed, and heirs according to the promise.

Joh 8:39 They answered and said unto him, Abraham is our father. Jesus saith unto them, If ye were Abraham's children, ye would do the works of Abraham. ... 41 Ye do the deeds of your father.... Then said they to him, We be not born of fornication; we have one Father, even God.
42 Jesus said unto them, If God were your Father, ye would love me: for I proceeded forth and came from God;... 44 Ye are of your father the devil, and the lusts of your father ye will do.