The confusion I think comes because Paul talks of 2 Israels-
Believing Israel (aka the Church, of which I am a part), and unbelieving Israel (physical unbelieving Israel). The Messiah is referred to as Israel
Isaiah 49:1-6
1 Listen to Me, O islands, And pay attention, you peoples from afar. The LORD called Me from the womb; From the body of My mother He named Me. 2 He has made My mouth like a sharp sword, In the shadow of His hand He has concealed Me; And He has also made Me a select arrow, He has hidden Me in His quiver.
3 He said to Me, " You are My Servant, Israel, In Whom I will show My glory." 4 But I said, "I have toiled in vain, I have spent My strength for nothing and vanity; Yet surely the justice {due} to Me is with the LORD, And My reward with My God." 5 And now says the LORD, who formed Me from the womb to be His Servant, To bring Jacob back to Him, so that Israel might be gathered to Him (For I am honored in the sight of the LORD, And My God is My strength), 6 He says, "It is too small a thing that You should be My Servant To raise up the tribes of Jacob and to restore the preserved ones of Israel; I will also make You a light of the nations So that My salvation may reach to the end of the earth."
Note in verse 3 particularly, that Messiah is referred to as "Israel". He is the vine and/or tree, and in Romans, that is what we are grafted into.
Additionally, Paul uses the remnant concept to show this, that in every age, physical Israel consisted of those who were God's people and those who weren't. The ones who were were called the remnant. Remember that those who left Egypt, many were idolaters (Ps. ) and/or rebellious (Ps. ) and almost everyone died in the desert. Paul invokes the remnant terminology in
Rom. 11:1-7
1 I say then, God has not rejected His people, has He? May it never be! For I too am an Israelite, a descendant of Abraham, of the tribe of Benjamin. 2 God has not rejected His people whom He foreknew. Or do you not know what the Scripture says in the passage about Elijah, how he pleads with God against Israel? 3 "Lord, THEY HAVE KILLED YOUR PROPHETS, THEY HAVE TORN DOWN YOUR ALTARS, AND I ALONE AM LEFT, AND THEY ARE SEEKING MY LIFE." 4 But what is the divine response to him?
"I HAVE KEPT for Myself SEVEN THOUSAND MEN WHO HAVE NOT BOWED THE KNEE TO BAAL." 5 In the same way then, there has also come to be at the present time a remnant according to God's gracious choice. 6 But if it is by grace, it is no longer on the basis of works, otherwise grace is no longer grace. 7 What then? What Israel is seeking, it has not obtained, but those who were chosen obtained it, and the rest were hardened;
So if you are part of His (Jesus) body, you are part of the remnant of Israel that was chosen by grace.
Did you know that the 2 that survived of the Exodus generation (Joshua and Caleb), were a Hebrew (Jew) and a Gentile? Josh. 14:14 tells us that Caleb was a Kennizite.
Did you know that Jacob adopted two Gentiles into his own family when he blessed Ephraim and Manasseh? (Joseph's wife was a Gentile). Reading that passage, it states that Ephraim would become a multitude of Gentiles!! The rabbi's had problems with this passage as in many passages where Gentiles are included without explanation (so the rabbi's labored to explain why God didn't seem to make a distinction between Gentiles and Jews).
Jesus also referred to this-
John 10:16 -
"I have other sheep, which are not of this fold; I must bring them also, and they will hear My voice; and they will become one flock with one shepherd." (and NO, he is not talking about the Mormons

)
This is the same thing Paul is talking about in Romans 10-
Rom. 11:16-26
16 If the first piece of dough is holy, the lump is also; and if the root is holy, the branches are too. 17 But if some of the branches were broken off, and you, being a wild olive, were grafted in among them and became partaker with them of the rich root of the olive tree, 18 do not be arrogant toward the branches; but if you are arrogant, remember that it is not you who supports the root, but the root supports you. 19 You will say then, "Branches were broken off so that I might be grafted in." 20 Quite right, they were broken off for their unbelief, but you stand by your faith. Do not be conceited, but fear; 21 for if God did not spare the natural branches, He will not spare you, either. 22 Behold then the kindness and severity of God; to those who fell, severity, but to you, God's kindness, if you continue in His kindness; otherwise you also will be cut off. 23 And they also, if they do not continue in their unbelief, will be grafted in, for God is able to graft them in again. 24 For if you were cut off from what is by nature a wild olive tree, and were grafted contrary to nature into a cultivated olive tree, how much more will these who are the natural branches be grafted into their own olive tree? 25 For I do not want you, brethren, to be uninformed of this mystery--so that you will not be wise in your own estimation--that a partial hardening has happened to Israel until the fullness of the Gentiles has come in; 26 and so all Israel will be saved; just as it is written, "THE DELIVERER WILL COME FROM ZION, HE WILL REMOVE UNGODLINESS FROM JACOB."Note in verse 25 he says that the the Gentiles that come in are part of 'all Israel' - "and so all Israel will be saved".
If they are separate, why does Jesus say they will be one flock? BEP, this is not a salvation issue (it doesn't make you saved or unsaved), but it is about the nature of our salvation. I know what I am saying is different than what you are taught, but that doesn't make me wrong and you right, or vice versa. I think this is what the forum is for, as iron sharpens iron, and we compare our understanding with each other, and with the Scripture text. I would like to see clear scripture references that state that there are two bodies, and one is Israel, and one is the Church. Remember, the word church was a term used to refer to Israel (as in Stephen's speech before he was stoned in Acts 7). In my reading, everywhere that Paul talks about the body, the next thing me uses is an Israelism (in the list in my post to MalkyEL).
I have a book coming from BigD on the subject, so I hope to better understand the position that he is taking, although I don't know if it is the same one you endorse.
Peace and blessings
-oneBook