Are Christians in the Age of Grace under the New Covenant?
No. The New Covenant is made by God with the house of Israel and the house of Judah, not with the Body of Christ. Believers in the Age of Grace enjoy its benefits—the work of the cross, resurrection power, the Holy Spirit's coming—but they are not in a covenant relationship; they are members of the Body of Christ.
Les explains from Jeremiah 31 that God promises a New Covenant with the house of Israel and with the house of Judah. Many theologians claim that believers in the Age of Grace are under the New Covenant, but Les says that reading the Bible in plain English shows this cannot fly, because God makes this covenant with Israel and no one but. Christians cannot be in a covenant that God makes with Israel. Instead, they enjoy 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 of this Age of Grace—yet 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.
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: