Replacement theology is a doctrine of demons

Gentile believers are grafted into Messiah Yeshua and become part of Israel. They are *NOT* grafted into a tribe of Israel. They are gentiles - wild branches - that become part of Israel by ADOPTION - they are NOT the "ten lost tribes". Those that teach the gentile believers are the "ten lost tribes" have bought into a weird mixture of replacement theology (IE: british Israelism), dominionism, and calvanism (IE: only those who are of the "lost tribes" can be saved) This is contrary to what scripture teaches.

The foreigners are still foreigners (wild olives) and were NOT grafted into a tribe:

Exodus 20:10 but the seventh day is a Shabbat for Adonai your God. On it, you are not to do any kind of work — not you, your son or your daughter, not your male or female slave, not your livestock, and not the foreigner staying with you inside the gates to your property.

Everyone - Jew or gentile - irregardless of "tribe affiliation" can be saved:

Mark 16:15 And he said unto them, Go ye into all the world, and preach the gospel to every creature.

This is NOT the 'lost sheep of Israel' this is about the GENTILES:

John 10:16 Also I have other sheep which are not from this pen; I need to bring them, and they will hear my voice; and there will be one flock, one shepherd.

The apostle Paul contrasts gentiles and Israel. He also states the gentiles - the wild olive branches - are grafted in AMONG the natural branches - they are NOT grafted ONTO the other branches - they are grafted into Yeshua AMONG the native branches and they maintain their identity. The idea of grafting another variety into a tree is to have a different variety not so that it will change into the original variety :

Romans 11
13 For I speak to you Gentiles, inasmuch as I am the apostle of the Gentiles, I magnify mine office:
14 If by any means I may provoke to emulation them which are my flesh, and might save some of them.
15 For if the casting away of them be the reconciling of the world, what shall the receiving of them be, but life from the dead?
16 For if the firstfruit be holy, the lump is also holy: and if the root be holy, so are the branches.
17 And if some of the branches be broken off, and thou, being a wild olive tree, wert graffed in among them, and with them partakest of the root and fatness of the olive tree;
18 Boast not against the branches. But if thou boast, thou bearest not the root, but the root thee.
19 Thou wilt say then, The branches were broken off, that I might be graffed in.
20 Well; because of unbelief they were broken off, and thou standest by faith. Be not highminded, but fear:
21 For if God spared not the natural branches, take heed lest he also spare not thee.
22 Behold therefore the goodness and severity of God: on them which fell, severity; but toward thee, goodness, if thou continue in his goodness: otherwise thou also shalt be cut off.
23 And they also, if they abide not still in unbelief, shall be graffed in: for God is able to graff them in again.
24 For if thou wert cut out of the olive tree which is wild by nature, and wert graffed contrary to nature into a good olive tree: how much more shall these, which be the natural branches, be graffed into their own olive tree?
25 For I would not, brethren, that ye should be ignorant of this mystery, lest ye should be wise in your own conceits; that blindness in part is happened to Israel, until the fulness of the Gentiles be come in.
26 And so all Israel shall be saved: as it is written, There shall come out of Sion the Deliverer, and shall turn away ungodliness from Jacob:
27 For this is my covenant unto them, when I shall take away their sins.
28 As concerning the gospel, they are enemies for your sakes: but as touching the election, they are beloved for the fathers' sakes.
29 For the gifts and calling of God are without repentance.