Supersessionism is the view that the New Testament Church supersedes, replaces, or fulfills the nation Israel’s place and role in the plan of God. In other words, it is asserted that there can be only ‘one’ chosen people of God at one time, and since those who believe in Jesus/Yeshua now are ‘the elect’, then, ipso facto, Israel/Jewish people who do not believe in Him are no longer still chosen for the purpose for which God chose them. (This has many layers for discussion, but this is not the place for that; but consider this: YHVH chose the Children of Jacob to be His ‘special people’, yet also chose the Tribe of Levi for a unique calling with a better inheritance than the rest of his brethren.)
It’s as if the Church and Israel become merged into one without being distinguished, or else split into two components: Jewish believers become the Israel of God; Gentile believers constitute the Church. Another variation is to say the Jewish people are still God’s Chosen People, but Israel as a nation has no more relevance.
The modern return of the Jewish people as a reconstituted nation in its ancient and promised homeland has been a goad that is hard to kick against.
The response of the ‘supercessionists’ reminds me of Martin Luther, who turned on the Jews when they did not fit his expectations. May God remove the veil from the nearly blind among Christians, just as we pray for Israel’s partial blindness to be healed when the Jewish people believe the truth and are set free.