I believe Christendom came to this doctrine out of necessity from the human perspective: "Without a nation of Israel how could prophecies be fulfilled?"
Then in 1948 Israel was reborn as a nation. But this didn't seem to change Christendom's perspective of the fulfillment of prophecy.Who has heard such a thing? Who has seen such things? Shall a land be born in one day? Shall a nation be brought forth in one moment? For as soon as Zion was in labor she brought forth her children. — Isaiah 66:8
It seems the end result of Christendom's doctrine that 'the church' is Israel comes down to no one believing the 144,000 are literal Israelites.
I'm curious if there are Christian's out there who, like me, believe Israel is still the fulfillment of prophecy.