Israel is not cast away. Romans 11:1-10
Accepting the Gentiles through grace does not mean God excludes Israel. On the contrary, He earnestly desires that they might be saved.
However, Israel is no longer favored over the Gentiles. From the beginning, God accepted believers by His grace. Just as God told Elijah he wasn’t the only prophet left, Paul says there remained a remnant whose hearts were not hardened toward Him.