Why can't believers in the Age of Grace be under the New Covenant?

The New Covenant is made specifically with the house of Israel and the house of Judah, not with the Body of Christ. Since God makes it with Israel alone, believers in the Age of Grace cannot be participants in that covenant; they only enjoy its benefits, such as the cross, resurrection, and Holy Spirit, without being in a covenant relationship.

Les explains from Jeremiah 31:31 that God will make a new covenant, and it will be with the house of Israel and with the house of Judah. Although many theologians claim that believers in the Age of Grace are under the New Covenant, reading the Bible in plain English shows that this cannot fly, because God makes this New Covenant with Israel and no one but. So how can believers be in a covenant that God makes with Israel? They cannot be. Instead, they are enjoying the benefits of everything God did to fulfill this covenant with Israel: the work of the cross, the power of His resurrection, the coming of the Holy Spirit, and all the blessings enjoyed in this Age of Grace. But it is not a covenant relationship. Believers are members of the Body of Christ, He is the Head, and they are in the Body, but they are not in a covenant relationship. God is going to make a New Covenant with the house of Israel.

Jeremiah 31:31

Behold, the days come, saith the LORD, that I will make a new covenant with the house of Israel, and with the house of Judah:

Why could God not make the new covenant earlier?

Scripture indicates God could not make the new covenant until the prior covenant had run its course. The Abrahamic covenant still needed fulfillment—Israel as a nation in its land under God's government—and the law covenant from Egypt had to be completed. Only then would Israel's spiritual condition permit the new covenant.