The church is Israel - for the first dozen or so years after Pentecost the only members were Jews. This early church is the remnant of Israel (and a larger remnant than was left after the destruction of the first Temple). Then, over time, people “of the Nations” were incorporated into (grafted into) this Jewish remnant. We are the fulfillment of that promise. Paul is very clear on this in Galatians, as is the writer of Hebrews. The old Covenant is fulfilled (completed) in Christ, with a new Covenant (Testament) taking its place. This is the mystery hid for ages until the fulfillment of time. And it is glorious in our eyes!
Whoever taught you that bunk needs to read scripture.