The doctrine that I hate most among those held by classic dispensationalists is their assertion that history consists of a series of attempts by God to save sinful men, attempts which failed over and over, to be replaced by Plan B (then C, D, E, and F). In contrast, Reformed theology holds that a plan of salvation was determined before the creation, with the Father decreeing that a church would be saved, the Son undertaking to purchase that redemption, and the Holy Spirit undertaking to apply that redemption to those who are saved. This agreement is known by various terms, such as the covenant of redemption or the intra-trinitarian covenant.There is nothing wrong with either term, but I generally use the latter to avoid confusion with the covenants of works and of grace, which are between God and men.
There are a number of places where we find references to this conversation among the Persons of the Godhead, especially in the Psalms and in the Book of Isaiah. I want to look at a passage in the latter, 49:1-10.
We can tell that this passage is about Christ because He applied it to Himself in the New Testament. Consider the remark in Is. 49:2: "He made My mouth like a sharp sword." Compare that to Revelation 19:15: "From His mouth comes a sharp sword with which to strike down the nations." Or compare Is. 49:9, "Saying to the prisoners, 'Come out,' and to those who are in darkness, 'Appear'" to Jesus's words in Luke 4:18, "He has sent me to proclaim liberty to the captives and recovering of sight to the blind...."
In this portion of the covenant, what does the Father promise? First, He promises that in Christ He will be glorified, v. 3. This will be through His work of redemption (John 12:28), through His people (Matthew5:16, John 15:8), and by answering their prayers (John 14:13).
Then He is promised success in the restoration of Israel (Is. 49:5): "He who formed Me from the womb to be His servant, to bring Jacob back to Him; and that Israel might be gathered to Him." We are still waiting for the fulfillment of that promise, but it is repeated in the New Testament (Romans 11:25-28). Yet, the Father says that this honor is not enough (v.6): "It is too light a thing that You should be My servant [merely] to raise up the tribes of Jacob and to bring back the preserved of Israel." If restoring the elect remnant of Israel is too small of an honor, how shall the Father increase it? "I will make You as a light for the nations, that My salvation may reach the end of the earth." As honored as Christ would be to receive the elect Jews, the Father extends that promise to elect Gentiles, as well! This is the "fullness of the Gentiles" of Romans 11:25. It is, indeed, a glorious promise! To make it even more certain, the Father had already made that promise in Psalm 2:8: "Ask of Me, and I will make the nations Your heritage, and the ends of the earth Your possession." And the Scripture describes that promise as fulfilled in Revelation 11:15: "Then the seventh angel blew his trumpet, and there were loud voices in
heaven, saying, 'The kingdom of the world has become the kingdom of our
Lord and of His Christ, and He shall reign forever and ever.'" See also, for example, Isaiah 2:2-3 and Micah 4:1-2.