In the end of Moses life he has a song where God prophesies about Israel falling away
and God spiring up jealousy by going to the gentiles
Isaiah and the Song of Moses start the same way and both speak of a rebellious Israel and Isaiah
appears to build on the same. Moses spoke of a future falling away. Isaiah speaks as though it is here.
So what's with all this God yelling to a people who are not my people who aren't even seeking him 'here I am! here I am! In a Christian view the people is the believing church, predominantly gentiles.
'A People yet to be Created' in the Psalms - Three songs pointing…
Hosea also brings this up in Hosea 1:9-10.
The point of this is really two-fold. In the new Covenant God will do two things; cut off unregenerate old Israel and accept in gentiles who are regenerate.
What this means is that old Israel is "not my people" because God cut them off; but both Jews and gentiles who are regenerate (through the new birth experience) will be called "my people" who previously were "not my people".
Jews are not my people because God cut them off and they disregarded His Covenant and gentiles are not His people because they never had a Covenant to begin with. Why were Jews cut off? Because they broke His Covenant and because they that are still in the flesh cannot please God at all. God must have a spiritual nation.
So, in the new Covenant you have a spiritual nation that is comprised of Jews and gentiles. Unfortunately for old Israel; God no longer accepts a physical unregenerate nation. All, both Jews and gentiles must be born again of the water and the Spirit to please God. (John 3:3-5) Because God desires a nation that will worship Him in Spirit and in truth. Not in oldness of the written rituals of the Mosaic Law. (John 4:21-24, Romans 7:6)
So both Jews and gentiles are now one in this spiritual nation Israel. The "nation" that God makes the Jews jealous with is
not a physical nation. It is a spiritual nation. This is why it's called a "foolish" nation. (Deuteronomy 32:21) Because according to the understanding of this world; a "spiritual" nation doesn't make sense. The world only understands physical, tangible nations like the English or the Dutch or whatever. If you tell a worldly person you are part of a
spiritual nation that doesn't have any land on earth; but is waiting for a city made by God (Hebrews 11:10) they will just role their eyes and think you're a nutter.
So God makes the Jews(the ones that are not regenerate) jealous with a "foolish" nation. But, He does so on purpose in order to goad them into repentance. (Romans 11:11-12)