John Chrysostom
Since there were probably many fainthearted people who did not believe His promises precisely because Christ died, Paul, wishing decisively to refute this opinion, presents an example drawn from common custom.
What custom is this? For this very reason, he says, one must be confident.
Why? Because testaments are valid and receive force not while the testators are alive, but after their death.
Therefore he begins his discourse thus: "He is," he says, "the Mediator of a new covenant."
A testament is made on the last day before death. This testament makes some heirs and deprives others of inheritance.
So here also, concerning the heirs Christ says: "Father, I desire that those whom You have given Me may be with Me where I am" (Jn 17:24).
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Again, concerning those deprived of 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 says what they must receive and what they must do.
So here also, after countless promises, Christ gives them a commandment: "A new commandment I give to you" (Jn 13:34).
Again, a testament must have witnesses. Hear once more what He says: "I bear witness to Myself, and the Father who sent Me bears witness to Me" (Jn 8:18).
He also says of the Comforter: "He will bear witness to Me" (Jn 15:26). And when sending the twelve Apostles, He said: "Bear witness before God" (Jn 15:17; 1 Tim 5:21).
"And for this reason," Paul says, "He is the Mediator of a new covenant." What does "He is the 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 those entering into 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 fallen away 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. He transmitted the Father's will to us, and in doing so He underwent death.
We had offended God and deserved to die, but He died for us and made us worthy of the covenant.
Thus the covenant became firm, since it was no longer made with the unworthy.
At 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 events of the Old Testament, which especially could affect them.
But they will say: no one died there. How, then, was that covenant confirmed?
In exactly the same way. How? There also there was blood, just as here there is blood.
Do not be surprised that there it was not the blood of Christ, for there was a type there. Therefore the Apostle says: "Therefore not even the first covenant was inaugurated without blood."