What does Old and New Testament mean?
This is not really very well understood in Big Eva (Broad-Evangelicalism). People normally think it means the old covenant of law and the new covenant of grace, but that’s not where OT / NT titles come from.
The Westminster Confession of Faith (WCF 7.4) explains that the terms Old and New Testament has to do with last things, that is the consummation of all things. It has to do with a “Testator”, that is Jesus Christ as the one whose death involves the bequeathing of an everlasting inheritance.
The Old Testament is where we first find out about this inheritance, particularly with Abraham, where he and his Seed (That is Christ) and all his seeds (that is all who believe and are saved in Christ) inherit the land (Gen 13:14-16). Now it was a bit difficult for Abraham, because he was also told that he was going to die, being gathered up to his fathers (Gen 15:15), but that he was also going to still inherit the land (Gen 15:7). We’re told in Hebrews that Abraham had already considered that God could raise the dead. (Heb 11:19)
We find out from Paul in Romans 4:13, that this inheritance is actually not just Canaan, but that was just a type, or symbolic placeholder for the whole world. We find out also by Paul in Galatians 3 that we New Testament believers in Christ are also in on this inheritance, because we’re Abraham’s offspring, heirs according to promise. (v3:29)
So, what about all the Mosaic Covenant and all the rigid laws, and having to obey to be able to stay in the land and the requirement to be perfect to live and all of that?
Well in addition to the law being God’s perfect standard for righteousness, all that was part of the typology to. The mixed Mosaic covenant was more of a subservient covenant to the prior Abrahamic promissory grant form of covenant. Israel is the type of the new creation people of God. The land was a type of Eden for Christ as the second Adam to fulfill the terms of the first merit based covenant, and by fulfilling the law to earn the inheritance. By doing so Christ saved us as our federal head. Our salvation and part in Christ’s inheritance is not works (merit) based, but it is grace alone through faith alone in the gracious / promise / grant type of covenant that God had first made with Abraham (really begun in Gen 3:15), and administered through types and promises.
The law also served as a teacher or tutor to lead them to Christ. Paul says in Gal 3:22 that “the Scripture imprisoned everything under sin, so that the promise by faith in Jesus Christ might be given to those who believe.”. Those not in Christ and therefore in the covenant of grace (promise) are still in Adam and under a works (merit and performance) based covenant with God and are therefore cursed under the law.
The administration of the types of the covenant of grace in the OT were in addition to things like the promises made of an inheritance. So we see types of a sacrificial substitute in the animal skins that God clothed Adam and Eve with (Gen 3:21), and the Passover lamb that covers those who have faith to put it over their doorposts so that God’s judgement passes them over (Exo 12:1-28), in circumcision, which is a sign of a cutting off or death sign removing of sin (Gen 17:9-14), and in various other ways in the OT.
In the NT it’s administered more simply through word and sacrament, where we see Baptism as a bloodless washing away sin death sign where the floodwaters of judgement went over Christ’s head, as if being drowned in a judgement flood (Luk 12:50) , and us with him(Rom 6:3), and yet he was raised for our justification (Rom 4:25) , and us with him, to eternal life. (Rom 6:9)
So, we see two principles, administration and substance across both testaments. That the administration changes between testaments, but the substance (which is Christ) remains the same. In other words, it’s all about Christ who by His works, His perfect righteousness under the covenant of works secures everything for the people of God, and how God grants everything to us in Christ in the covenant of grace through the instrument of faith alone. So the good news across the whole bible is about a grant, or inheritance for those who are righteous in God’s eyes by faith, that is just by believing in Christ as he is presented administratively, and through promises in the Word.
We find out more as the narrative progresses in the bible into the New Testament that we Gentiles are also invited in and able to enjoy this inheritance because as we rest in Christ, we’re granted eternal life, and we’re Abraham’s offspring who are adopted into the household of God, a royal family, and we’re to live in the New Jerusalem, which fills the New Heavens and the New Earth forever. So, Old and New Testament is about an eternal inheritance, in and because of Christ and what He has secured for us. Really the whole bible has this focus and trajectory of the consummation of all things, it’s all about Christ’s testament, based on His death in this present evil age which results in an eternal inheritance in the glorious age to come.