Hebrews 9:16

"there must also of necessity be the death of the one who made it," This is what disturbs them, the death of Christ. For how, he says, can the one who has died give us the promises? Therefore, he turns and shows that we can only receive the inheritance and the promise if Christ has died. For if he had not died, he would not have made a will, so that we might inherit; (for it is indisputable that a will is in force only after death;) nor would we be worthy of an inheritance at all, as long as the enmity remains unresolved.

Inasmuch as the apostle says to the Hebrews, "A will takes effect only at the death of the one who made it," he therefore asserts that, with Christ's death for us, the new covenant has become valid. Its likeness was the old covenant, in which the death of the testator was prefigured in the sacrificial victim. Therefore, if one should ask how it is that we, in the words of the same apostle, are "children and heirs of God and fellow heirs with Christ," since of course the inheritance is made valid by the death of the deceased and since an inheritance cannot be understood in any other way, the answer is this: he himself having in fact died, we have become heirs because we were also called his sons. "The sons of the bridegroom," he says, "do not fast while the bridegroom is with them." Therefore we are called his heirs, for he has left the peace of the Church, a peace which we possess in this life, in our possession through faith in the divine plan of salvation revealed in time.

It was probable that many of those who were more weakly would especially distrust the promises of Christ because He had died. Paul accordingly out of a superabundance introduced this illustration, deriving it from common custom. Of what kind is it? He says, "indeed, on this very account we ought to be of good courage." On what account? Because testaments are established and obtain their force when those who have made them are not living, but dead. "And for this cause," he says, "He is the Mediator of the New Testament." A Testament is made towards the last day, [the day] of death. And a testament is of this character: It makes some heirs, and some disinherited. So in this case also: "I will that where I am," Christ says, "they also may be." (John xvii. 24.) And again of the disinherited, hear Him saying, "I pray not for" all, "but for them that believe on Me through their word." (John xvii. 20.) Again, a testament has relation both to the testator, and to the legatees; so that they have some things to receive, and some to do, So also in this case. For after having made promises innumerable, He demands also something from them, saying, "a new commandment I give unto you." (John xiii. 34.) Again, a testament ought to have witnesses. Hear Him again saying, "I am one that bear witness of Myself, and He that sent Me beareth witness of Me." (John viii. 18.) And again, "He shall testify of Me" (John xv. 26), speaking of the Comforter. The twelve Apostles too He sent, saying, "Bear ye witness before God."