If Jesus covers all those who accepted Him in faith in the O.T. and for all time, why do the Jews have to go back to keeping the law during the Tribulation and Millennium? And, do the Gentiles need to do this, too?
Please understand that faith in a Redeemer who would die for the sins of the people is a Biblical truth that was established before the foundations of earth were laid (1 Peter 1:18-20). And through all the age of man this faith has been accompanied by the requirement to obey God’s laws. The Age of Grace did not abolish this requirement, it only suspended it temporarily. As soon as the Church is gone it will be necessary for obedience to accompany faith again. Ezekiel 40-46 make this clear for Israel and Rev. 14:12 confirms it will also be necessary for Gentiles.
Instead of asking why people will have to go beck to keeping the Law, a better question would be how did the Church alone escape this requirement? The answer is in Ephesians 2:7, “That in the ages yet to come He might show the incomparable riches of His grace, expressed in His kindness to us in Christ Jesus.”
People who have grown up in the Church without much exposure to the Old Testament, have no idea how uniquely blessed and favored of God we are. No other group has been or will be given the seal of the Holy Spirit as a deposit guaranteeing our inheritance (Ephes 1:13-14), adoption into the royal family of God (Galatians 4:4-7), separation of behavior from belief in the matter of salvation (Ephes. 2:8-9), a place at the side of Jesus in the heavenly realms (Ephes. 2:6), etc.