Medieval church?
Justin Martyr was writing around AD 155, and literally titled the argument:
“Christ is King of Israel, and Christians are the Israelitic race.”
He said Christ is Israel, and Christians - be they Jew, or Gentile - are the “true Israelitic race".
Irenaeus, around AD 180, says flat-out that Abraham’s seed is “the Church,” and that those of faith are Abraham’s children.
That’s quite a trick for a doctrine supposedly invented by the "medieval Church".
You’ve missed the date by roughly a thousand years.
Also, Let’s see…
Irenaeus, c. AD 180 — “His seed is the Church.”
Justin Martyr, c. AD 155 — “Christ is the Israel and the Jacob” … Christians are the “true Israelitic race.”
Justin Martyr — "Rejecting Christ means rejecting the One who sent Him: “he who insults and hates Him, insults and hates Him that sent Him.”
Cyprian, c. AD 250 — argues that the former covenant people lost their privileged standing through unbelief and that “the Christians had succeeded to their place,” coming from “all nations and from the whole world.”
Augustine — To be a “true Israelite”, one must follow the “true living Jesus” into the true promised inheritance.
Irenaeus — The prophets sowed concerning Christ, but “the Church reaped” and received the fruit.
So as far as historic Christianity goes, you don’t have a leg to stand on and you have a thousand-year hole where your argument is supposed to be
