In the context it is quite clear that the Israel of God is the church. The letter has already said there is now neither Jew nor Greek in the Christian church, so how cane there be an Israel of God identified as Jews?
The very context has said that there is now neither circumcision nor uncircumcision but a new creation. How then can there still be circumcision which is what Israel of God as meaning Jews would imply. Paul was not that stupid.
But it is wrong to say that that is the only verse. Ephesian 2 makes plain that Gentiles who have become Christians have become part of Israel. You WERE strangers (non-Israelites) --- you are NO MORE strangers (non-Israelites).' 'You were alienated from the commonwealth of Israel -- you are now fellow-citizens'. It could not be clearer.
Furthermore
1 Peter 2.9 makes it crystal clear. Speaking to Christians who had been Gentiles he speaks of them as 'an elect race, a royal priesthood, a holy nation, a peculiar people' just as God had called Israel 'a peculiar possession to Me --- a kingdom of priests, a holy nation (
Exodus19.5-6).
Jesus Himself taught that Christians would be Israelites. They would abide in the TRUE VINE (the true Israel of which Jesus was the representative), they would be a c
ongregation built on Jesus Christ, they would b a part of the new nation (
Matt 21.43). Jesus WAS representative Israel (out of Egypt I called my son, and baptised on Israel'sbehalf) . Thus to be in Christ is to BE an Israelite (not a Jew).
Rom 11.12-24 unquestionably demonstrates that the olive tree (God's chosen name for Israel -
Jer 11.16) was composed of ALL who believed in the Messiah. Thus they were part of Israel.
I have shown you otherwise.