In two places in Isaiah, the Father promises a people, a posterity, to the pre-incarnate Son. In Isaiah 42:6, He says, "I am the Lord; I have called you in righteousness; I will take You by the hand and keep You; I will give You as a covenant for the people, a light for the nations." The promise is even grander in Isaiah 49:6: "It is too light a thing that you should be my servant to raise up the tribes of Jacob and to bring back the preserved of Israel; I will make you as a light for the nations, that my salvation may reach to the end of the earth." The first could refer just to the Jews, but the second expands the promise to the Gentiles, as well, to give a fuller glory to the Son.
The Son responds in Isaiah 8:18, "Behold, I and the children whom the LORD has given me are signs and portents in Israel from the LORD of hosts, who dwells on Mount Zion." And again in Psalm 22:22: "I will tell of your name to my brothers." As the Father glorifies the Son with a posterity, so does the Son glorify the Father to that posterity. These last two verses are explicitly applied to the Son in Hebrews 2:12-13.
In the New Testament, we see Jesus claiming these promises of the Father in the Gospel of John. He refers to "those You have given me" in John 10:29: "My Father has given them to Me" [i. e., His "sheep"]. He makes similar remarks several times in John 17:4, 6, 9, 11-12, and 24.
These verses describe what I have called the Intra-Trinitarian covenant. It is also called the covenant of redemption. There is more to that covenant than I describe here; I am merely describing one aspect of it. It is the basis of our salvation. The Father elected a church from all eternity, and gave it to the Son for redemption. I don't describe it here, but the Holy Spirit is also involved, undertaking to apply the redemption to the elect. This covenant, however, as much as we benefit from it, is not about us. it is about the glory that each Person of the Trinity gives to the others. I compare it to life insurance. Since it only pays upon the death of the party insured, he receives no benefit from it; rather, the benefits go to the beneficiaries, who are third parties to the contract. In the same way, the elect are the beneficiaries of the intra-Trinitarian covenant: we were not consulted, nor is it for our glory, but from it we receive redemption from our sins.
This covenant goes against two false doctrines. The first is that the love of God and the atonement of the cross are intended for everybody, but not necessarily effectual to anyone. The second is far more heinous, i. e., the doctrine of the Oneness pentecostals, who deny the Trinity, deny the person of the Father and the Holy Spirit. All three Persons of the Trinity are, and have always been, involved in our salvation. Without the Trinity, therefore, there can be no one saved.
This doctrine is described in the Westminster Confession of Faith (VIII:1): "It pleased God, in His eternal purpose, to choose and ordain the Lord Jesus, His only-begotten Son, to be the Mediator between God and men, the prophet, priest, and king; the head and Savior of the Church, the heir or all things, and judge of the world; unto whom He did, from all eternity, give a people to be His seed, and to be by Him in time redeemed, called, justified, sanctified, and glorified."