See what the Jews didn't know and the world in general, and even most religionist are still ignorant of, is that when God gave Gospel Promises to Israel under the old covenant, as it pertained to the new covenant such as Jer 31:31,33-34
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:
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.
That included in this House of Israel and Judah was a remnant of men from amongst the gentiles that are fellow-heirs of the same body ! That was a Mystery !
Amen to #1291 and #1293 !
Christ whose Name is Israel Is. 49:1-3, and His Chosen People in union with Him from Everlasting to have forgiveness of sins and an Eternal Inheritance Eph. 1:4-11 are One Body Heb. 2:11: Israel Is. 45:17.
Therefore they comprise the House of Israel, a Spiritual Seed Gal. 3:16, 29, the Children of Promise, whether jew or gentile, and are given Faith Rom. 4:16.
So those who received this prophecy Jer. 31:31, 33-34 were Spiritual Israel who had Faith given them in their Head 1 Pet. 1:10-11, but were not aware of the fact that the Israel that was Promised Salvation included also gentiles and were of the same Body; hence the Mystery.
But merely being Abraham's physical descendants of the flesh, these are not the children of God, and Christ didn't die for them !
That is, They which are the children of the flesh, these are not the children of God: but the children of the promise are counted for the seed.