Romans 9

In proving that the prophetic Word of God had "not taken none effect" but that there had indeed been the fulfilment of God's promises - promises "pertaining" to his "brethren, my kinsmen according to the flesh: Who are Israelites...and of whom as concerning the flesh Christ came" - Paul explained that "they are not all Israel, which are of Israel".

It meant not all Israelites were true Israelites. 

No mention in his argument yet of Gentiles.

He only said not all Israelites were true Israelites.

He said that, in order to assert that the seemingly limited fulfilment which they had seen and were seeing amongst Israelites, of those promises which pertained to them, was nonetheless the fulfilment of those promises for them.

This was because only believing Israelites among the Israelites were the true children of God.

Paul asserted all this before even mentioning Gentile believers!

He asserted that there had indeed been the fulfilment of God's promises pertaining to Israel, in Israel and amongst Israelites - in the experience of the believers among the Israelites.

Then, after justifying God's actions in that outcome of Israel's promises in the following verses, only after all that does Paul finally then add a comment about Gentile believers, when he added that the chosen vessels whom God called were "not of the Jews only, but also of the Gentiles".

And in proving the legitimacy of the Gentiles also experiencing that same promised-salvation which had been promised Israel and which had already been fulfilled in and for Israel and which believing Jews were already experiencing, Paul proceeded then to quote a prophecy which was about Gentiles specifically.

So the Church, the one new man, in which there is now neither Jew nor Greek, the heavenly Jerusalem, mount Sion, came into existence through that prophetic and now past-historical scheme.