Speaking of Christ, the author of Hebrews writes:
Therefore he is the mediator of a new covenant, so that those who are called may receive the promised eternal inheritance, since a death has occurred that redeems them from the transgressions committed under the first covenant. For where a will is involved, the death of the one who made it must be established. For a will takes effect only at death, since it is not in force as long as the one who made it is alive.9:15-17
An eternal inheritance may be given “since a death has occurred that redeems them from the transgressions committed under the first covenant” (v.15). Christ’s death redeemed His people. It did not potentially purchase anything, but rather definitively purchased all it was intended to acquire. Only Christ redeeming death qualifies those who are called for an eternal inheritance.
Christ obeyed God perfectly while on earth, thereby earning for Himself an eternal inheritance of joy. However, this inheritance could not be enjoyed by us until He died (vv.16-17), until His new covenant was inaugurated with blood (v.18). For this reason, we may refer to Christ as our testator. The Westminster Confession of Faith reads thus: “This covenant of grace is frequently set forth in the Scripture by the name of a testament, in reference to the death of Jesus Christ the testator, and to the everlasting inheritance, with all things belonging to it, therein bequeathed” (7.4). We benefit from God’s promises only by the death of Christ.
This glorious reality was central to early church theology. The Letter of Barnabas was composed no later than the early 2nd century (perhaps in the first century). In this letter, we see the surprisingly robust covenant theology of the church shining forth.
Moses indeed received it, but they were not worthy. But how did we receive it? Learn! Moses, being a servant, received it, but the Lord himself gave it to us, to the people of inheritance, by suffering for us. 5 And it was made known that both they might complete their sins and we might receive the covenant of the Lord Jesus through the one who inherited it, who was prepared for this reason, that by making known himself what had already been paid over to death, our heart, and had been given over to the lawlessness of deception, redeeming from the darkness, that he might make a covenant with us, a covenant in word. 6 For it is written how the Father commands him having redeemed us from the darkness to prepare for himself a holy people.Letter of Barnabas 14:4-6
We are prepared a holy people by the redemption of Christ (v.6), at the command of the Father (v.5). We receive the promises of God only because of our benefactor and testator, the Lord Jesus. From the beginning, Christ’s church has put full stock in His substitutionary atoning death upon the cross. May we continue this tradition.
 Rick Brannan, tran., The Apostolic Fathers in English (Bellingham, WA: Lexham Press, 2012).