When the majority of the Israelite people in Paul's generation rejected Jesus as Messiah, did God give up on them? Absolutely not. God continued to work with the faithful remnant (see previous sermon in this series) and remained faithful to his promise to Abraham by reserving a future time of restoration.
But what did God do in the meantime? The era in which we now live is known as the Church Age, is was proceeded by the era of the Mosaic Covenant, and will last until the establishment of the Messianic Kingdom. During this era God's work with the Gentiles, aside from its straightforward purpose of leading countless people to salvation, is also a method of using the jealously of God's covenant people (i.e. the descendants of Abraham, that is, Jews) to draw them toward faith in Jesus. Why? Because God's love for them, as a people called to his purpose, remains.