The Church - prepared for in the Old Covenant
The remote preparation for this gathering together of the People of God begins when he calls Abraham and promises that he will become the father of a great people. Its immediate preparation begins with Israel's election as the People of God. By this election, Israel is to be the sign of the future gathering of All nations. But the prophets accuse Israel of breaking the covenant and behaving like a prostitute. They announce a new and eternal covenant. "Christ instituted this New Covenant."
Source: Catechism of the Catholic Church (Vatican.va)