Paul explains the national rejection of Christ by Israel by referring back to the teaching in the Old Testament about the remnant. He shows that God has always had a remnant, a small group of true believers. Israel's national rejection does not mean that every Jew rejected, because God always has a faithful few. However, because of the national rejection by Israel of their Messiah, their hearts have been darkened judicially by God. Because they would not believe, now they cannot believe.