Are believers in the Age of Grace under the New Covenant?
No. The New Covenant is made with the house of Israel and the house of Judah, not with believers in the Age of Grace. They enjoy its benefits—the cross, resurrection, Holy Spirit—but are members of the Body of Christ, not in a covenant relationship.
Les teaches that while many theologians claim believers in the Age of Grace are under the New Covenant, the plain reading of Jeremiah 31:31 shows God makes this New Covenant with the house of Israel and the house of Judah. He asks how believers could be in a covenant God makes with Israel; they cannot be. Instead, believers enjoy the benefits of everything God did to fulfill that covenant with Israel—the work of the cross, the power of His resurrection, the coming of the Holy Spirit, and the blessings of 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, not in a covenant relationship. God will make the New Covenant with the house of Israel, and verse 32 says it is not according to the earlier covenant.
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: