John Chrysostom
Since there were likely many fainthearted people who did not believe His promises precisely because Christ had died, Paul seeks to refute this opinion decisively. He presents an example drawn from common custom. What custom is this? For this very reason, he says, one ought to be confident. Why? Because testaments are valid and receive force not while the testators live, but after their death. Therefore he begins his discourse in this way: "He is," he says, "the Mediator of the New Covenant." A testament is made on the last day before death. This testament makes some heirs and deprives others of the inheritance. So also here, concerning the heirs Christ says, "Father, those whom Thou hast given Me, I desire that where I am, they also may be with Me" (Jn 17:24).
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Again, concerning those deprived of the inheritance, hear what He says: "I do not pray for all, but for those who believe in Me through their word." In a testament there is what concerns the testator and what concerns those who receive the testament. It states what they must receive and what they must do. So also here, after countless promises, Christ also gives them a commandment: "A new commandment I give to you" (Jn 13:34). Again, a testament must have witnesses. Hear again what He says: "I bear witness of Myself, and the Father who sent Me bears witness of Me" (Jn 8:18). He also says of the Comforter, "He will bear witness of Me" (Jn 15:26). And when He sent the twelve apostles, He said, "Bear witness before God" (Jn 15:27, 1 Tim 5:21). "And therefore He," Paul says, "is the Mediator of the New Covenant." What does "He is Mediator" mean? A mediator is not the master of the matter in which he mediates. The matter is one thing, and the mediator another. For example, at a marriage there is a mediator, but he is not the bridegroom. He only assists the one entering marriage. So also here. The Son became Mediator between the Father and us. The Father did not wish to leave us this inheritance. He was angry with us and displeased with us, since we had departed from Him. The Son became Mediator between us and Him and entreated Him. See how He became Mediator. He took upon Himself both accusation and justification, transmitted the Father's will to us, and in doing so underwent death. We had offended God and ought to have died, but He died for us and made us worthy of the covenant. Thus the covenant became firm, since it was no longer concluded with the unworthy. In the beginning God made a covenant with us as a Father with children. But when we became unworthy, there should have been not a covenant, but punishment. Why, then, says the Apostle, do you boast in the law? It brought us to such a sinful condition that we would never have been saved. If our Master had not died for us, the law would have helped us not at all, because it was weak. Yet he proves this not only from common custom, but also from the events of the Old Covenant, which could especially influence them. But they will say, no one died there. How, then, was that covenant confirmed? In exactly the same way. How? There was blood there, just as here there is blood. Do not be surprised that there it was not Christ's blood. There it was a type. Therefore the Apostle says, "Therefore not even the first covenant was confirmed without blood."