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"For neither is circumcision anything, nor uncircumcision, but a new creation. And to those who will walk by this rule, peace and mercy be upon them, and upon the Israel of God. (Gal 6: 15-16).
To the apostle, the Israel of God is synonymous with the new creation. Unlike the "old" Israel under the Old Covenant which had an old and vanishing way of approaching God, the "new" Israel is under the New Covenant and has access to God through a new and living way (Heb 10:20). It is comprised of both Jew and Gentile who have been made into one "new" man in Christ (Eph 2: 15) with a "new" commandment to love one another (John 13:34). Entrance into this "new" creation comes through the "new" birth which is brought about by grace, through faith in Christ and is not based on fleshly descent, social standing, or gender (Gal 3:28).
Thus in Paul’s mind there are two Israel’s that exist simultaneously in time and have always existed in this manner. That is why he can state categorically that
"For they are not all Israel who are who are of Israel" (Romans 9: 6 NKJV).
Try using this type of phraseology with any other nation or nationality and see how foolish it would be. Imagine saying that, "they are not all Arabs who are Arabs"! Or how about, "They are not all Eskimos who are Eskimos". It would make absolutely no sense whatsoever. But it makes perfect sense to use Paul’s language because there are two different Israel’s – one "after the flesh", the other "after the spirit". Look again at how Paul further differentiates between the two in that same letter to the Romans:
"For he is not a Jew who is one outwardly; neither is circumcision that which is outward in the flesh. But he is a Jew who is one inwardly; and circumcision is that which is of the heart, by the Spirit, not by the letter; and his praise is not from men, but from God." (Romans 2: 28-29).
We see the exact same truth referred to by none less than our Lord Jesus who in His revelation to the churches of Asia Minor addresses the churches at both Smyrna and Philadelphia in the following manner:
"I know your tribulation and your poverty (but you are rich), and the blasphemy by those who say they are Jews and are not, but are a synagogue of Satan." (Rev 2: 9).
"Behold, I will cause those of the synagogue of Satan, who say that they are Jews, and are not, but lie – behold, I will make them to come and bow down at your feet, and to know that I have loved you". (Rev 3: 9).
There simply is no other way to explain this than to realize that not everyone who can trace his or her fleshly descent back to Abraham can be considered to be a Jew and thus a part of Israel. Otherwise we are left with the insane possibility of someone claiming to be of one race or nationality all the while belonging to another! Further proof of this truth is attested by our Lord’s remarks concerning Nathaniel in the gospel of John.
"And Nathaniel said to him, ‘Can any good thing come out of Nazareth?’ Philip said to him, "Come and see.’ Jesus saw Nathaniel coming to Him, and said of him, ‘Behold, an Israelite indeed, in whom is no guile!’ Nathaniel said to Him, ‘How do You know me?’ Jesus answered and said to him, ‘Before Philip called you, when you were under the fig tree, I saw you.’ Nathaniel answered Him, "Rabbi, You are the Son of God; You are the King of Israel.’ Jesus answered and said to him, ‘Because I said to you that I saw you under the fig tree, do you believe? You shall see greater things than these.’ And He said to him, ‘Truly, truly, I say to you, you shall see the heavens opened, and the angels of God ascending and descending on the Son of Man.’" (John 1: 46-51).
The Lord Jesus calls Nathaniel an Israelite indeed or in truth. In contrast to the vast majority of the nation, Nathaniel was a true son of Jacob whom had seen the heavens opened centuries before with the angels of God ascending and descending upon a ladder. In the same manner in which God had changed this man of guile and duplicity into a true child of God and renamed him Israel, so too would Nathaniel be counted among those who had been recipients of the same work of divine grace in their heart? Clearly then it is no stretching of the Scriptures to categorically state that among the vast numbers of those who make up national Israel and call themselves Jews, there are those who belong to God and those who do not but are rather children of the devil. What else does the term, "synagogue" of Satan refer to?
This is precisely what Jesus told the Jews in the eighth chapter of John’s gospel.
"I speak the things which I have seen with My Father; therefore you also do the things which you heard from your father.’ They answered Him and said, ‘Abraham is our father.’ Jesus said to them, ‘If you are Abraham’s children, do the deeds of Abraham. But as it is, you are seeking to kill Me, a man who has told you the truth, which I heard from God; this Abraham did not do. You are doing the deeds of your father.’ They said to Him, ‘ We were not born of fornication; we have one Father, even God.’ Jesus said to them, ‘If God were your Father, you would love Me; for I proceeded forth and have come from God, for I have not even come on My own initiative, but He send Me. Why do you not understand what I am saying? It is because you cannot hear My word. You are of your father the devil, and you want to do the desires of your father.’" (John 8: 38-44).
There! That should keep you busy.